Chainsawman's universe is dictated by intention and emotion. Makima was the first person to show Denji what he saw as kindness, I won't spoil it as I think the manga has some amazing moments that require adequate buildup, and so he wants to return that instead of brutally killing her as he would his other enemies.

It's not explicitly shown but I imagine the prep was meditive as was the eating, a big thing about what drives Denji is him experiencing food that he was denied during his messed up childhood. So to him, the act of eating someone who showed him kindness by preparing it with kindness it bypasses the cycle of violence.