If you look, he's not much older.  His ex-girlfriend was present.  The guy clearly had some sort of a breakdown, which anyone is capable of.  
It's a tragedy, but the fact that the guy is a police officer doesn't matter to me.  I don't like that the story is told in a way that would whip people up into paranoia about who is protecting their neighborhoods.  Officers are only human, and I doubt that any degree of screening could have identified him as a risk and averted this.  
Also, he was off duty.  It's not as though he was using his badge to validate his actions.