1. According to Christians, God is the transcendent standard for morality--based not outside Himself but founded in his own nature.
2. Can a world be conceived of in which murder is the Good founded in the nature of a creator god?
3. If it's conceivable, then it's possible murder could have potentially been ethical. Which is absurd.
4. If it's not conceivable, then one is a applying a moral standard outside of herself and her creator god.