Why are you two even arguing over this? Whether he left out of honor or to be evil really doesn't matter to how the fight went, and Loeb admitted he didn't know much about DS other than what he heard from Walter Simonson. I doubt he spent much time studying the essence of Darkseid's character. The bottom line is that DS was leaving, Superman suckered him, and DS easily countered when he attempted to capitalize on the sucker-punch. Why he was leaving doesn't really matter.
I will say in regards to Doomsday Year One that the narration makes it clear that a slugfest between them would've been a hell of a fight, and DS doesn't leave until Steppenwolf lets hm know the chemicals have been destroyed. Doomsday also turns his back and walks away rather than attempting to attack DS while his back is turned. They appeared equally respectful of each other in that instance.