Yoongi had dissociative identity disorder. 

In other words, he was the main character from Dr. Jekyll and Mr. Hyde. He had two personalities, one who was murderous, ruthless and had almost no emotion and the other who was gentle, caring, and sometimes lazy. The disorder caused him to switch between the two without having any memory of the other while in that state.

No one knew he had this disorder besides his older brother, Jin. Every time he blanked out, Jin was there to protect him and make sure he did nothing wrong. Suga, his other self, never seemed to hurt Jin, he trusted him.

This was the way it was, until Jin broke that trust.