Yeah, mom and dad, we hear these kind of sad excuses all the time, with the single exception that you haven't described him as an 'aspiring rap artist trying to get his life back together'.
What you have there is the human equivalent of a rabid dog. It doesn't matter if he was a cute little tyke once, you let him grow up to be dangerously crazy, so blame yourselves in part for what happen to him.
As far as a justification for executing him (I prefer a rope, but that's just me), I prefer an old Texas legal standard - he's a man who just deserves killing.