Souma isn't entirely irredeemable. He's cocky and arrogant, but he genuinely wants to do better. He also jumped in to 'save' MC without hesitation. That should be enough reason for MC (who also became op by getting plenty of support in the previous world) to want to help him. That this is also a Perfect way to cover his tracks by helping Souma defeat the demon by himself and thus keep his hands clean is a nice bonus. Besides, he's already helped straighten out someone far less worthy in the first few chapters, so it's within character.