Quoting Addi's Momma:" I am sure I would have beat the crap out of him but never murder. Also how did the father have non-life threatening injuries but both of his kids died? Were they buckled in correctly?"
I think what probably happened is that something snapped in the guy's head. Obviously he's dealing with a lot of grief losing two children in such a senseless way due to the irresponsibility of someone else. Is he justified? Yes, I think he is justified in wanting to kill him. To me anyone who takes someone's life has something definitly wrong with them emotionally. In this case (unlike many others) we can see the cause. Doesn't make it right but does give him a reason.