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.