Forgive yourself for your thoughts. Anyone would feel the way you do if treated badly. Especially if their partner doesn't protect them from this treatment. Focus on your actions. Are you polite and respectful? If so, congratulate yourself. That's hard to do when others treat you badly. As far as wishing him ill, one thing that keeps me wishing the SDs in my life happiness and success is that i know if they are happy and successful, they won't be a burden on my SO and therefore won't be a burden on me if we get married. Does that make me a saint? Nope, but it works for me. 

I tell my DH that before my feelings about SD's change, I will have to see them have changed, improved actions and behavior over an extended period of time. Not just 2 or 3 days of improvement here or there or throwing a bone our way from time to time but long-term, continuously appropriate behavior. Until then I have no faith in SD's whatsoever. Actions speak louder than words. While we're at it, I would also like an apology.

For myself and many other's, I think the main reason we as SM's have a lot of negative thoughts about our SK's is because our SK's are really god awful people. We think they are sneaky, rude, ill mannered, raised by wolves, the product of guilty/permissive parenting, lack social graces, passive/aggressive and so on because THEY ARE. They've never once said "sorry" for any mean, nasty behavior or word they have said to us. At least mine hasn't.

Perhaps my SDiablas's are a little better with strangers and outsiders than they are with us (or BM) but they still tend to be solitary loners. They can't get or keep a boyfriend. They complain to DH about turbulant peer friendships. For example, OSD's friends got sick of her being overbearing, judgemental and intense.

I feel guilty about how much I dislike my SK's too but my goal is to allow myself to have those feelings without judging them. There is nothing wrong with having feelings. Try to give yourself grace to feel what you feel and hopefully let it pass. (It hasn't passed for me yet but maybe one day?)

Try to find places and people with whom you can be your genuine, true self, the person that you want to be. That can be with your own friends and family or volunteer or get involved in something with someone, anyone, other than and away from your SK. Do something that you can do to remember and feel good about the generous, kind, loving person that you are.

My advice is that when thoughts about SK's or BM become too repetitive, consciously chose to think about something else or do something that distracts your mind from lingering on them.

Maybe you should find a new church?

From your post, you attend church, as I do. So, we have our "religion" in our ear, so to speak. In the area of forgiveness, the church has had serious mis-steps with it's "forgive and forget" mentality over the past decades. But that mindset is changing.

I'm going to suggest 2 books that have been instrumental in my healing from some serious toxic treatment over the past 17 years (and I hope help you with that unwarranted guilt you are experiencing). Forgiving What You Can't Forget, & Good Boundaries and Goodbyes by Lysa Terkeurst. Major points: #1. Forgiveness is commanded in our faith and it only takes one...the offended person. Reconcilliation takes TWO people willing to do the WORK to restore a broken relationship. Many relationships will never be restored and that is okay.  #2 Boundaries are GOOD; they don't have to be walls...just a good solid boundary. #3. I only give of my heart to those who have shown trustworthiness with what I have already given. 

Things got really bad here in 2018 and I haven't seen most of DHs kids since then. This past year, we've had a few interactions and I see more in the future. Still...I do NOT trust DHs kids and I will NOT subject myself to their behaviors ever again. As long as all parties are respectful to each other, that's all I want. It's not my place to look for the "good or bad" in them...just take them at face value and protect myself appropriately. We are humans that would never be friends in any other situation, so I go with "civil and superficial." I care about them as I care for all humanity....not wishing ill on anyone but we will never be close and personal. And I'm okay with it intelectually and spiritually.  The two interactions in our home - I was civil, offered drinks, said a few pleasantries and excused myself to do other things....letting DH visit with his kids. 

I hope some of my story helps you. I know those struggles you are having personally (with the guilt and emotions). Please know you aren't wrong or bad for your feelings and thoughts.....it's natural for what we've been through. Wishing you the best.

You were being disrespected by SS.  Someone who disrespect me I would feel the same. Just because he a SS Does not remove all of his responsibilities. Yo you and the world.  You just dintblove a SS because of his father.  He must earn it 

At some point in my life I came to the realization that emotions are good. That I was jealous of her past relationship.  She and her ex could  just go to McDonald fir a hamburger before chrildren.  Me. It's was a major thing, getting a babysitter, leaving instructions,  the babu=ysitter costed more then the hamburger.  That I miss the big wedding,  she had.  She had a big wedding , No one woukf pay for the second wedding . With three kids and no support, food was the major problem, not a wedding.  
 

I understand this completely. I have a similar situation with one of my adult SSs. He's insincere, doesn't apologize when he does horrible things, attacks - blames and shames. He moved out and boomranged back - it was painful. After he left the second time my numbness turned to red hot anger at his selfish and horrible behavior. He held me personally responsible for everything wrong in his life. It took years for me to process the anger and realize it was abuse. Afterall he was a child for much of the time I helped raise him but now he is a large grown adult male. His behavior hasn't changed but my attitude and how I conduct myself has. Not sure this will be the same path you are on and it's extremely hard when they are in the house but start figuring out how when he comes to adult age you set him free and set yourself free. I have a feeling there will be quite a bit of relief for you when he leaves and a sense of peace. Count down the days. Don't feel bad or guilty. This guy is takign from you and not treating right or respectfully. Spend time making sure he has a plan and is out. 

My experience is they don't really get better with age or mellow out - so far they've kind of stayed the same with their bad behavior but I am not subjected to it on regular basis. I also very happily opt in and out of things that don't suit me. I just stopped trying to be it all for them and I view them as guests that I respect but I don't place a lot of emotional energy into them. I also accept after over a decade that they don't have concern for me, love or even care. They don't and they probably never will. I am simply a vehicle to get what they want or an obstacle if they don't get what they want. 

I still have my moments of anger and frustration but I work hard to realize that this is a missed opportunity for them, less so for me. Give yourself a HUGE break and pour the love into you. 

You gift yourself joy by living your best life.  You can do all of that while being clear on what and who SS-17 is. He has shown you who he is. Believe him.  These types rarely if ever change.  Past behavior is the best predictor of future performance/behavior.  He has proven this. He will prove it again, more likely than not.

No need to beat yourself up over SS's behaviors. Even when he is on his temporary best behavior, he is still a liar and still a sneak.  Those are behaviors he has repeatedly perpetrated against you and who knows who else and how many of them.

Forgiveness does not mean you forget or that you continuously serve yourself up to their toxic behaviors.

There is nothing for you to forgive yourself of IMHO. He has earned neither forgiveness nor forgetting his repeat toxicity.   Faith does not mean you abandon intellect and reality.  So, don't.  

Our greatest gift as a species is intellect. Use it.

Don't doubt yourself and for sure do not tolerate him to undermine your own best life, joy, happiness, and confidence.  You know him, you know the truth. Make him live that truth.

Regarding your concern with your Church community. There are any number of scriptures that emplore protection from evil doers.  Remember intellect and free will.  Though not a scripture, God does protect those who protect themselves. So, do for yourself what God has indicated will be done for you. Psalm 140 is the primary biblical reference that I am aware of on dealing with dipshit evil people. So, deal with SS.  Help God help you.

IMHO of course.

Living well is also the best revenge. Enjoy both living your best life, and having that life exact a just and pointed revenge.

Gaslighting yourself -- I'm so glad you said that because it appears that this is exactly what you have been doing.  It's easy to do. And I agree with what everyone has said. It's so hard to be a good person, to truly want what is best, and then to be confronted with some of the worst, hurtful behavior in others. It's especially hard because people like us DO question ourselves. I hate that! Your reactions are normal.
I made the mistake of letting my guard down a number of times with my skids. At one point, SS was calling me to discuss his issues with a girlfriend. But after each time of thinking things had changed, believing that maybe there was a relationship there, my hopes were stomped on.
It's really hard to accept that some people just are the way they are, and that only long-term change will prove otherwise. But it's also such a relief to accept what you see as real and to move on from there. Disengagement has helped my mental health and sense of serenity enormously! 

Neuroplasticity (brain can adapt and change throughout life, depending on nurture/environment), genetics, early developmental experiences can be overridden - all these ideas took off so much that we're completely in denial about some people never changing no matter what. 
 

Popular psychologists/ psychiatrists and academics flood us with all these amazing stories of feral creatures becoming kind-hearted, intelligent, contributing members of society. Yet, this is a percentage of all feral kids born to feral parents. The ones that had all professional, world-class resources thrown at them. And even then only some succeed, while others go down the drain. All these stories are political, serving those selling a new method, technique or approach - we must remember that. 
It is easy constantly being bombarded by  these success stories to believe that if you do A, you will get B, but it's not so linear. Some people are genetically predisposed to little or no empathy, impulsivity, anger and inability to connect and no amount of love and kindness will put these qualities into them.