Continuity mistake: We discover that because time is non-linear in the Further, it was Josh's spirit who was the one knocking on the door in the original film. However, the knocks we see Josh make in this film don't quite match up with the original film. In this film, he knocks many times frantically, whereas in the first film, we only hear a few faint knocks in rounds of three. (The film tries to mask this by inter-cutting footage from the first film to illustrate the point, but it still doesn't quite match up).
Continuity mistake: When Josh finds Renai on the floor and takes and places her on the couch her head is facing upwards, but in the next shot her head is tilted and facing the camera when Josh smells her, without her head moving.
Continuity mistake: In the first Insidious film, when Lorraine shows adult Josh the photographs, the Lady in Black is seated in the car behind the young him. However, in the second film, when Lorraine shows the photos to young Elise, the Lady in Black seems to be standing in front of the car.
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.