Tucker: Bring it.
Young Elise Rainier: In my line of work things tend to happen when it gets dark.
Josh Lambert: I know what happened. I went in that place to get our son back and something evil followed me, who killed Elise.
Mother of Parker Crane: Don't you dare.
Answer: He answered that while chasing Renai, it was because at first he didn't really want to but his mom kept planting seeds in his head and telling him that if he doesn't he's going to die and towards the end you can tell he was very close to dying so he gave in and try to kill the family. Also throughout the whole movie it talked about the fact that his mother basically forced him to kill people.