Depends on what you mean by "corrupt". If he's still serving the his original god, then give him normal abilities, but maybe penalize his spell powers in some way to reflect the displeasure of his god with his weaknesses, be it alcoholism, promiscuity, gambling, or whatever. And make sure he has any appropriate skills (which will depend on which version of RM you are using and what options, if RM2) to match his "corruption". If you mean he has fallen to evil, give him some Evil spell lists (probably only to a low level, as he apparently didn't start out as an Evil Paladin). Again, there might be some skills you want to give him for thematic reasons. But otherwise, stat him out as needed for your game. If he's going to be the "boss" villain for a session, make him tough. If he's going to be the main villain for a series of sessions, make him *really* tough. If he's an NPC support character that the PCs are expected to redeem, make him useful but make sure he won't overshadow the PCs.