He wasn't trying to just see his kids, he was trying to take them away from the grandfather's house. I realize that the courts sometimes make a mistake in giving custody to the mother, but this guy sounds like a hothead with a bad attitude. Or maybe there was bad blood between the father and grandfather.
In any case a younger man has no justifiable excuse for attacking an 80 YO man with a recent pacemaker implant. You may have had a bad experience in a custody case yourself, and if so I can understand your attitude. But unless I knew more details about the situation I wouldn't jump to the conclusion that the father was justified in what he tried to do.
The father had already left the house and the grandfather had followed him out and killed him in the driveway.
So who was the hothead with the bad attitude?