I knew one from high school, in NH, I lived in a border town of Massachusetts (Lowell), the kid moved, got into gang life (or really just hanging out with junkies and criminals) around 14, ended up stabbing a Cambodian to death in an argument when he was 16.
When I moved to Pittsburgh, Pa after high school, working warehouse for a building my father was running, we had a prisoner work exchange, so I made friends with a lot of folks that were serving, but my father would give anyone a second chance (except convicted pedo's/rapists/domestic abusers, and I know that stuff is hard to screen, but other inmates and the coordinating PO that ran the program for work exchange would help us filter out). A couple of the supervisors served time for m1, and because the old man would give jobs to people that were trying to turn their life around, I have met a few over the years living there. My brother was really producing a lot of music at the time, and a couple of the rappers he worked with were crash test dummies, my blow dealer at the time had a "justifiable" because he was selling soap and passing it off as crills, the buyer pulled out on him, so he did him, pretty scummy. Not sure exactly if he served time behind fake dealing, a manslaughter charge, or what, but he did about 8 years behind it.
In Upstate, a kid that did a lot of music with my brother got picked up one day, out of the blue, and got put away on a m1 shooting. One of the nicest, most positive, humblest people I had ever met, we were all fukking shocked.
Growing up, until I moved to Pittsburgh, even after my friend from school, I thought 1 way about the kind of people that were capable of taking another life, but as I got to know more people, you never really know, just know there are a million different reasons someone might find themselves in that situation.