It'd be interesting if the past sins Leon wants to atone for involve killing monsters indiscriminately, before he realized that not all monsters are evil. Some are just wild animals doing what they need to do to survive. Some can be tamed to do the bidding of humans, which can be good or bad depending on that particular human's intentions. And some monsters, despite their inhuman appearance, have human-level intelligence and are likewise capable of choosing to be good or evil. Perhaps Leon had an encounter with an intelligent monster (maybe that dragon he's famous for fighting, maybe the slime chief he nearly killed) which made him realize that exterminating innocent monsters effectively made him a mass-murderer. That would make a nice dramatic reveal if Leon ever finds out Jelly is a slime impersonating a human, but instead of becoming hostile, he's overcome with remorse and confesses his past and begs Jelly for forgiveness.