>>713566217He had enough determination to save, load, and reset, but not enough to actually properly genocide the way you do. He could murder everyone in the underground, but wouldn't be able to permanently destroy everything and would just reset after he got bored of nothingness. Likewise, he knows the player will find the neutral ending unsatisfying and reset for a better one, which is why he's not scared of death after you beat him in the neutral route. It's the permanence of genocide that terrifies him.