Sometimes I don't understand the world

On Friday a former co-worker of mine committed murder/suicide.  It's been over a year and a half since I worked with him, but he worked as a front lanes manager for the first year and a half that I worked.  I used to joke with him, talk sports, each lunch....next to someone that committed murder.  And then killed himself.  I've never been in a situation like this before.  Some people at work are sad over his death, others saying things like "he killed someone, I don't feel sympathy for people that murder."  Me? I just have a stone in my stomach.  I find it nauseating.  But don't know what to feel.  But in the 24 hours since I heard I can't seem to stop thinking about it.  Honestly, I wish I didn't know.  I've always thought of people who do that as whack jobs.  Someone you'd say to your self "That person's totally unhinged."  And maybe I'd have thought that if I had seen him recently.  But I can't.  I still see him as a laughing, joking man.  And you think, if a person can spiral down that deep into the hands of evil - can everyone?  And it makes the world a scarier place for me today.