DustSans. This guy had to see his friends, family, and people he knew get slaughtered repeatedly over 300 times. The messed up thing was; only he remembered what happened. Every time the player reset, everyone forgets what happened. Except him. At one point, he broke. He thought in his deranged mind, if he got more EXP than the player, he could beat them. To get EXP you have to get kills...
DustSans killed HIS friends, and eventually...his own brother. In the end, nothing changed. The player still beat/killed DustSans. All he can do is relive his entire life...over and over...