Sanity

One of the experiments in the lab go wrong and bring sans from the future, who goes by the name demon, to the past. Demon is just sans but aged 28 and left completely insane after thousands of resets, leading him to do anything to try and stop the loop only to ultimately get stuck in worse loops with him now taking part in the killings and even sometimes enjoying them. He can be easily distracted. He burns through his energy stores extremely fast