If you mean Seth, he did it in order to make himself a powerful cursed weapon that would be able to actually harm a dead god's soul, which was the only way he would be able to stop an already dead guy like Osiris... Seth knew he and Osiris would meet again someday, so he prepared himself, and he thought only the powerful curse of destroyed human souls would have any effect on a dead god.
Oooooooohhh okie oksi okei .. That made sense .. Tnx
Anyway, it's the reason for at least SOME of what he did...
So ok.. But now what i want to know is why did he kill some people ..? There must be a reason or he's really want to kill bcs of his anger idk ..