I've struggled with a situation for some years now and I just need a reality check/kick up the arse/advice. Also sorry as long!
I spent years in a very violent relationship from Uni to my late twenties. I was completely messed up by it all (unsurprisingly) and had no idea I was in an abusive relationship until it was far too late. I will call him A
Eventually I left after he seriously injured me. Because my boundaries were so blurry, we stayed in touch via our Uni friendship group, although I eventually lost touch with them when I finally told them the truth (some stayed friends some didn't).
Fast forward 6 years and one of my closest friends from Uni (I will call her B) admitted that she was now in a relationship with A. I couldn't cope with it at all and our friendship ended. I was, however, still in touch with her sister (who I will call C) and her husband, (who I will call D) who I had been friends with for years. She wasn't overly happy about A & B but supported her as they were sisters.
It all came to a head a couple of years ago when C developed terminal cancer. All her old friends came back together to support her, and as a result I was thrown back together with A & B (now her DH). Just before she died, C asked me to promise that I would "watch B's back"; that she didn't trust A and wanted to make sure that, in her words, "someone who knows what he is capable of" kept an eye out for B. Admittedly she was at end-stage by that point and I don't know if she would have asked me if she hadn't been heavily dosed up, bless her.
In the intervening years, meantime, I got involved in a second abusive relationship (now divorced thank-god) and had years of psychotherapy.
I am now in a hideous situation of being back in touch with A & B as well as still good friends friends with D, (they live locally) and emeshed in a friendship group where I have to pretend that I think A is a lovely chap! D is very fond of, and loyal to B but through circumstance is also now very close to A and won't speak out against him (they all have children and babysit for eachother, go on holiday etc). I constantly get group e-mails and texts from them all and I'm finding myself getting more and more uncomfortable having to go along with the accepted 'narrative' that A really has changed, is a lovely guy and is one of my friends. (from what I can see and from the way he treats B, he hasn't actually changed at all - nothing obviously violent, but lots of EA)
I feel like I've let my boundaries get too blurry, I am so angry at myself for not having the guts to tell them all to get lost, (truthfully, I don't have that many friends). I also worry about B and her kids but am not sure if I am the right person to help them as it's affecting me in a really negative way. I hate the fact that me and A are now FB 'friends' (because it looks like either I lied in the first place or minimised the abuse). I did try defriending him a while back but kept getting calls asking why he's fallen off my friends list!
Arrrgh! I can't believe I've got myself into this utter mess. My fear is that if I walk away, then I will lose D who I am quite close to and like a lot as a friend. B keeps telling me that I am one of her best friends and how much she loves me. It's very confusing, but I am beginning to feel as though A & B are using my friendship in different ways, to validate A's image as Mr Nice Guy and B to convince herself that that everythings fine and she's not really married to an abuser.
Extremely well done if you've got this far, sorry it was so long and potentially confusing, but...
Do I just walk away from the whole thing and accept that I will lose them all but will retain some semblance of personal dignity and agency over my life, knowing that A is an abusive twat? Is it okay to break my promise to C? (I feel like it is but still feel guilty)
Please lovely vipers, give me a kick up the backside becuase this whole situation is making me feel horrible! 
