Dungeon Master needing a jerry springer's advice

[Venerable]

Tell him that either he needs to find a way to accommodate the other player's character choices or he needs to find a different game to play in.

Bingo! And I'd take that even further. Killing another player's character because their alignments differ is easy roleplaying. You say your friend's a good role-player, so challenge him OOC to find a way to work with the other character. "You must cooperate; now, why would your character do so?" That's a lot more difficult to RP, and a lot more rewarding. With luck, he'll rise to the bait.

This doesn't mean the end of intraparty conflict... but the conflict should serve the role-playing, not vice versa. Also, killing another player's character violates gaming rule #1: help others have fun.

I should know. For the last half-year my character's been adventuring alongside someone he'd kill if circumstances were even slightly different. Realizing that the characters have to work together has really improved my RP skills, and revealed new aspects of my character.