I don't discount the possibility that Rudi and Henri had a punch up earlier on, but something tells me that this was not a spur of the moment family massacre to kill witnesses. I think it was to get the family money, he had a long-standing sense of injury, being a narcissist, and entitlement, and had perhaps been exposed during the argument. That pirate game or cartoon or whatever it was might also have pumped him up, after he had the idea to murder them all.
The way the family was drawn in that night/morning was premeditated. He would attract the attention of his father or mother to ambush them, and deal with them one by one as they approached.
He is fortunate that Theresa and Marli didn't manage to escape and raise the alarm before he could attack them. If Theresa hadn't yet seen inside the bedroom she wouldn't have guessed what Henri had done in there and would have had no reason to try to escape as her first course of action. I think each of them intervened thinking they could diffuse a fight and then to help the others before any thoughts of running to safety could kick in. It sounds like Marli really put up a fight and I wouldn't be surprised if she pushed Henri down the stairs, but he came back.