Because he's fighting against gods. Right now, that guy isn't a threat, but if he were to come back in his other form then it would be impossible for Guts to even get near him without it.

[UGHHHHH SHE GIVES UP MAN there is no way she's spilling the beans and he won't believe herrrr]

I understand that the armor is dangerous. And I'm not trying to stop you from having an opinion -- you don't have to like it. But he will go after revenge no matter what anyone says, no matter if people think it's right or wrong or whatever. If I can help control the situation -- which I can -- then that's what I'm going to do.