Sylar is forced to relive his murders.
Penance Stare is a specialization some telepaths use to manipulate the cerebral cortex of the victim, forcing them to relive traumatic experiences. When eye contact is made, the victim immediately finds themselves in the grip of excruciating fear and pain, usually the same fear and pain they have suffered in their lifetimes. The more traumatic the incidents, the more powerful this ability becomes. While the victim is suffering from the effects of the stare, the traumatic memories replay endlessly before their minds' eye, carrying waves of crushing emotion along with them. The DNA Alternate can continue to subject the victim to their traumatic memories, possibly causing permanent psychological disorders.