I think Lawful Stupid is a bit harsh. Lawful Dim, perhaps. Self sacrifice is sometimes necessary. And not fearing for one's life - or fearing, but still willing to do what needs to be done - is brave, not stupid. In fact, I'd say if O-Chul believed he'd get out just fine would make him stupid.He's a neat paladin, but he's still getting himself killed in a lawful stupid way.
O-Chul is no match for Team Evil. But, if O-Chul can play decoy long enough for V to escape, then V can bring in the Group, and the Group can stop Team Evil.
Additionally, O-Chul's sacrifice would be a strong push to get MItB to Do the Right Thing.