Killer was suffering because he was separated from his crew. He was just a madman killing in hopes that one day he would reunite with them. I don't necessarily think his problems came from the SMILE itself.
The SMILE doesn't allow him to express negative emotions, but that doesn't mean it greatly corrupted his mind. If he cries inside, he's smiling outside. He's still having those thoughts which are a healthy part of human psychology. He's simply not allowed to express them.
Yasu manged to be strong with a SMILE. I believe Killer is at such a stage now.