Take page from Princess Bride. Kidnap him and then kill him on the border with some enemy troops. Since the two groups are already at war, no one will think that it could have been a third party. Course that might cause issues with the L/G members.
You could always not tell them, or just hire an assassin. Depending on how influential you are you might be able to blackmail him into going straight. Evil men like power and will do just about anything to keep it. Threaten to expose his crimes or slip a dagger between his ribs, and he'll come around. If he doesn't, meh, politicians are a dime a dozen.