@willyleeks: The kid talked shit to Max about his family getting murdered. Considering that event is the starting point of Max's life going downhill, him gunning the kid down was easy to see coming.
Max was a cop, and a damn good one. Having your life go downhill fast enough to break the sound barrier changes a person, but things like a sense of justice stick around. I just got past the machete scene, so maybe something happens later on, but so far it's clear Max still has feelings, and he means well. Just to give a few examples, his interactions with the IT guy, and the short bit with the graveyard employee.