Okay theory time!
The victim in his next life became a heinous murderer deserving of a death penalty but he was unaware of his past life. Only just before his death penalty, he was shown his past life. With his past life’s feeling of pure rage against a killer juxtaposed with his current life’s actions, he truly felt the true repurcussions of his action, which he couldn’t feel previously being a psychopathic killer.