Actually, that's not true at all. There are many injuries, both mental and physical, that can deprive someone of sound thinking. In addition to that, there is also shock. Several years ago, I remember of friend of mine being rear-ended a few cars ahead of me. The speed limit on the road was 30-35/mph and the guy behind him hit his brakes too late.
Luckily my friend only suffered a few bruises and scratches. He was in shock though as several times while the paramedics were working on him, he kept trying to get up to go back to his car while saying he had to get his cat. He didn't even own a cat and the next day he remembered the accident, but did not remember talking about the cat he didn't own.
The father will likely be questioned thoroughly and they will determine whether or not he acted out of revenge or if he was temporarily insane. Based on the story, there is no way any reader can surmise which really happened. Any guess at this point is just that .. a guess.
/reality