Depending on how you're coping with it, therapy may be something to consider. It's an odd feeling when someone you were once very close to does something so horrific.
Back in 2009, my mom killed her second husband by shooting him in the head as he slept and dragged the body out to the garage, where she left it for a couple of days. She was sentenced to 65 years in prison, because it was considered premeditated, as she purchased a freezer shortly beforehand with the intention of storing his body in there. She thankfully stopped short of dismembering his corpse with an axe when she found that his body wouldn't fit into the freezer whole and turned herself in.
Keep in mind that she did this 2-3 days after Mother's Day that year. (That year was a complete nightmare in general, actually, as it was the same year in which my cousin and my older brother died.) I called her on Mother's Day, and everything seemed totally fine. I found out about it when my dad called me a couple of weeks later. It turns out that one of my brothers was Googling the names of family members for fun and came up with an article about our mom killing her husband. I was getting ready for work when my dad called, and I had absolutely no way how to respond. I ended up just going to work and spoke to my supervisor about it.
My supervisor was like, "Are you okay?"
I was like, "Yeah, I guess so. I mean, it's not like he killed her."
I really didn't know how to feel, though. Over time, I learned that her husband was really controlling and manipulative. It's a shame that things came to the point that they did, but I honestly feel very little inclination to stay in touch with her anymore. I haven't written to her in a long time, which may seem callous, but it got to a point that I just didn't even feel like I was having an exchange with my mom. She seemed like a completely different person. Not that the act itself had immediately changed her, necessarily, but rather that I don't really feel that she ever took proper responsibility for the act and seemed really dismissive of it.
Also, I have to say that I absolutely HATE working on Mother's Day now. I'm always really distracted at work on Mother's Day, because I'm continually and uncontrollably running through all of the various conversations that I might have with people in response to simple questions like "Did you get anything for your mom for Mother's Day?" or "Are you taking your mom anywhere for Mother's Day?" I hate to lie about anything, even something so trivial, so I won't just claim that I have anything planned. There've been years when the subject just doesn't come up, but it doesn't stop me from playing through all of these scenarios in my mind until my shift ends, because I'm not even comfortable talking to random people about it. I'm not just gonna blurt out the fact that my mom's in prison or that she killed her husband, as I don't think many people will even know how to react to something like that. It's something that people who know me reasonably well know, but it's something that I like to make sure I don't have to just blurt out and leave as part of a brief and incomplete conversation.
Obviously, my dad also had a lot to think about after this happened, because that very well could've been him, as he was similarly very controlling back when my parents were still together.
Best of luck in coping with this, in either case. It's super-weird when something like this happens, and it's a completely different feeling from experiencing the death of a loved one or just about anything else people could imagine. It's really unsettling how someone you know - especially if you think he/she wouldn't harm a fly - ends up doing something like this.
