He couldn't keep his control when Dark wanted to play. No matter what he did, no matter how many times he screamed in his head and tried to stay sane. When Dark wanted something, there was no stopping him. Because Dark was him. The other side of him that killed, that made things wrong. The side that he hated but couldn't get rid of.
Dark was too integrated into his personality. Whenever he played video games, Dark was the side that killed, the one that was determined to win, to do everything possible to eliminate all of the competition. When he played with Chica, Dark was the one that wanted to keep her forever, never let anyone else touch or hurt her. When he livestreamed, Dark was the one that secretly wanted to steal all of the money they donated, so he could use it for his own well-being.
He didn't want Dark. But he had no choice.
"You can't escape me."