Oh hey so this happened with my mom and her boyfriend. He just went away for a 30 year sentence. I still remember the urgency in her voice when she called me the day after it happened, the funeral for the victim's family, and all the legal back and forth that went on these last 3 years. PM me if you want.

It's absolutely not your fault; you couldn't have stopped this. Sometimes terrible things happen. The what-ifs will persist, but it's up to you to tame them. It's already happened, and now you're living in the aftermath. Try taking a walk before bed or a hot shower or just some ice cream because right now, you just need to do little things to alleviate. Take it one day at a time, step back from it all every now and then and focus on your own self-care. You know you can choose to be there to support him in whatever capacity you feel most comfortable with. Just gotta take care of you, too :)