We live in a society of laws, and that's a much better place to be than where anyone can enforce summary 'justice' on anyone else. Two wrongs don't make a right, particularly if the second one is taking someone out of the world. So yeah, the law should be applied. He should probably get the mandatory minimum sentence for murder since there is clearly provocation and so on, but he shouldn't get off without serving a sentence for murder, because we don't want to encourage vigilantism.