Are you for real.
He had no idea if there was a burglar in his bathroom. He had no idea were his girlfriend was, he had no idea who was in the bathroom, he had no idea if someone had broken into his house.
Why not open the door? Why not try and locate your GF, Why didn't he leave the house, Why didn't he call an ambulance. No better to shoot first and ask questions later.
You do that.
If he believed it was a burglar, he still has to take responsibility for his actions. Again, he had no idea who was in the bathroom, the fact his girlfriend wasn't in bed beside him might have been a clue to who was in the bathroom, opening the door to the bathroom might of helped, calling the Police or the estate security would have helped. By using his gun he took the law into his own hands.