As funny as it would be to learn Legosi is a... well, a bad dude, would be some really funny shit. But I don’t think Louis mutilated himself for this, because nothing has indicated he would need to and Legoshi made it known he’s not for flesh and such. Then I had the thought that it was Kyuu’s dead body, but that’s just weird as well.
If I had to take a super big swing here.... it’s rabbit blood, but specifically tied into Legoshi mentioning the dumb promise Haru gave to Melon.