SPOILER 

Reiner has a split personality.

One is the humorous joking side and the other is a dangerous "warrior" side.

He developed this other side of him when he witnessed Marcel(Bertolt and Reiner's childhood friend meant to go on the mission to destroy the wall)being eaten by Ymir.

When he goes to his warrior side he becomes ruthless- such as his decision to kill Marco.

Reiner is unconscious when he goes into his warrior state and forgets everything he did within that time.

Bertolt is the one keeping Reiner's warrior side from coming out.