Sure, he's not the first person in the world who's had his girlfriend's parents disapprove of him. But murdering them? Has life gotten so cheap that murder is now a first choice of settling agreements? How long can such a society survive?
It is called a crime of pasion, unlike animals humans have always had it, pasion that is.The girl was probably his first love and he was probably hers. I can imagine dad telling him on the way out the door to never call or see her again, to him it was unbearable. That is not a defense, there is none, but a probability to his mental condition at the time.