Life can be hard, and regardless of the fact that OP's school acquaintance has to face the music for his own actions and bears responsibility for them I do feel a bit of sympathy for him.
Obviously I don't know the details, but it sounds like this guy's not been happy with his life for some time, and has not felt or been able to make necessary changes. That'd come to a head when he knowingly put himself, his career (surely no way he'll work as a paramedic again after this), and other people at risk in order to go and live a temporary make-believe life with his mistress, escaping from the awful situation he'd just created with his actual wife/family. I don't think happy well adjusted people do things like that.
Does give you pause for thought wondering what led the schoolboy you were familiar with to end up in a situation like that. Maybe 'pathos' is the right word.
I guess I've had similar thoughts about someone from a few years below me at school that I was acquainted with who later stalked a girl for some time and stabbed her when she turned him down. And similar feelings of pathos about someone I knew from uni who died with his wife while on holiday last year (both in early 20s).
Life is a funny old thing, and you can never know what's going to happen next or what's really going on in anyone else's head.