Character doesn't really interest me. People can be extremely inconsistent. It's to the point where it's a standard joke that the neighbors of serial killers describe them as being nice people. So I personally see no inconsistency with either the supposed nice kid going bonkers or the supposed good cop turning murderous.

To me, the interesting questions are your second one plus the question of why the officer started shooting instead of leaving the scene, potentially returning with backup, since he was still in his car.