I think it's a little harsh to call the OP "evil" and blame her by saying she made a "bad choice." 

Not to say it wasn't a bad choice. Marrying him anyway has clearly led to a lot of bad feelings, but what's done is done. I don't think many people would deal well with the man they were with since high school knocking up a drug addict while they were "on a break", then years later being forced to be the primary caregiver to the child who likely has physical/mental problems from being born addicted and was probably raised very poorly by said drug addict. It's probably a nightmare and she is clearly in a dark place. It sounds like she was "over it" to an extent but having to be his fulltime caregiver and the stress of the quarantine may be bringing back some of that darkness she felt in the early years. I can't say that i wouldn't have hoped for a miscarriage in that situation and neither can anyone else who hasn't been there. She needs support, not harsh words. For all we know, that may be the difference between a positive outcome and, god forbid, a situation like what happened with the Gannon S. story. 

OP, i'm sorry you are going through this. The feelings you are having do not mean you are a bad person but they do indicate that you are severely stressed and something needs to change. Please do whatever it takes to get a break from this child. Your husband absolutely must take over the primary parenting for now. Be honest with him about the difficulty you are having, focusing on behaviors and situations that you can't handle, as opposed to saying you dislike the child. You must take a step back and really decide if this situation can be made into something you can handle long-term. Maybe if your husband steps up more it will work. If not, leaving the marriage may be the best option. And that is an option. You aren't trapped in this exact situation forever. 
 

After reading some of the comments and understanding how you feel, long story, I'd say stop doing anything for the kid. If he wants to be a dick then he can do everything himself, his own laundry, he can make his own meals. He's 11, he's gotta understand you're in charge, and your husband needs to get that into his thick skull too. Don't take either of their shit. I sure as hell wouldn't. I also understand wishing someone would miscarry. I wished for that too. I get the why and I don't blame you at all. Also, I'd do a DNA test, that kid might not be your husbands. Don't cling to that for hope. It's a simple possibility. Drug addicted mother? I doubt she even remembers how many people she slept with. 

If you need to talk my inbox is always open. 

 You have to ask for CS from your DH for a kid that lives in your marital home?  Really?  Everything DH makes and every penny of CS that BM pays are part of your marital resources, you should be getting it all and not have to ask DH for money to raise his children. Including the ones you share with him genetically.  You are also the bread winner.  You own DH's ass if you choose to.  Make sure he clearly understands this.  Outline his life paying a pile of CS, alimony and then having his toxic spawn full time with no help.

And DH does not get to inject his prior relationship breeding experiment into YOUR family vacation.  You hold all of the cards so use them.  Inform, don't discuss, DH that your SS will be spending that time with either of his GPs or his BM and not going on vacation with your family.

And.... DH nor SS get to stipulate how SS will be parented when YOU are the full time parent. You set the standards of kid behavior and performance in  your home and you apply effective consequences and increasingly unpleasant age appropriate discipline if SS fails to comply.  You are the parent to your own BKs and if SS represents a risk to how you raise and parent your children then SS can spend his time in an isolated space writing tens of thousands of sentences. Any behavior he perpetrates that is a bad example to your own younger children should realize an increasing state of abject misery as his life experience until he catches a clue.  Give DH an idea of how much he will be paying in CS and alimony (you should nail his ass for the years you have spent raising SS) if he does not immediately and permanently get in line on parenting the Skid.

The only bad choices I see in any of this is that your DH decided to dip his wick in the putrid gene pool that BM represents and his repeated bad choices in parenting the result of his poor choice of the first gene pool he frolicked in.

Since this kid is an incubus to your happiness, go teacher on his ass and deal with him as you would an ill behaved student.  Isolate him as much as possible and confront his poor behaviors with zero tolerance until he catches a clue. Make sure daddy hears every sordid detail of his eldests crappy behavior and that you give DH clear instructions on the discipline he will pick up when he is home. Again, zero tolerance.

Whether daddy is home or not. You are the adult in the mix. DH certainly isn't.

Good luck.

smh

I am reading through all of this and I absolutely get your frustration.  And the resentment.

I agree you should take a break, breathe, rejuvenate yourself.  When you come back, these issues will still need to be addressed.  This is a very complex siltation with a lot of layers and I don't know that there is a catch all solution.

Have you considered family/couples therapy?  It sounds like you and your husband have some old and new issues to work out to get back on the same page.  It also sounds like this child is dealing with a lot- who knows what he was exposed to with his mother.  Now he is in a household where he is likely aware, if only to some degree, that he is the outsider.  I am sure there is also a sense of abandonment.  No matter how bratty, selfish, and disrespectful he is, he is a child.

I am finding that a lot of people on this site are rather jaded.  Perhaps I am naive; I have only been given the stepparent responsibilities for a couple of years now.  I wouldn't give up on a motherless 11 year quite yet.

Just my two cents.