Despair exists in a Groundhog Day type of situation. Remembering everything when no one else does. They don't have the brute strength of many of the other monsters in their Underfell timeline, making just straight up killing people not really something he can do. So what does he do? Each reset, he experiments. They learn everyone's patterns. What everyone starts doing. Each incident that occurs. Each moment of vulnerability. Each moment where they could strike and easily take down even the toughest of monsters. And each time they fail? They fight to the death with reckless abandon. They know that if they lose, they'll just wake up in Sans's bed again, as if it never happened. And with new skills and knowledge to use next time. They grow more silent, knowing that anything they say won't be remembered, nor will change the outcome.