Jongin is in love with firrrreeeeee
He did not directly kill his mother. Fire was started, he could've saved her, he didn't.
He enjoys getting rid of useless people
Neglected and abused as a child by mother
Mother was bipolar and beat Jongin
Sometimes Jongin returned the favor

Jongin watched as his mother turned the knob, which enraged the fire dancing atop the stove. The blue fi

Kyungsoo is almost sociopathic, only feeling hate (to something he dislikes) and pleasure (in manipulation or hurting other people physically)

"Well... When the only things you know are hate and pleasure, and you gain pleasure in getting rid of what you hate... And you just happen to hate a lot of people... Let's just say others won't very much like the outcome."

Taemin is an actor, plays his role enough for him to be his role. When playing, he doesn't feel fear and feels that he has all the power he needs. 

"Y'know, Junmyeon is payed to act like he cares. But, I think he's acted it out enough to become what character he's supposed to play."