I don't understand what Ainz wanted to achieve with that duel. If he wanted to respect the dood, he would've at least let him fight a little instead of insta-killing him like that. If he didn't care, he should've just ignored the duel and killed them all.

You can argue Ainz used his full power out of respect, but that doesn't hold water because of the vast power difference. You don't "respect" a dumb puppy trying to pick a fight with you and then stomp its head into the ground to kill it. In fact, Ainz never needed to kill him. He could've just used some paralysis or sleep spell and be done with it.

Also, the conclusion those 2 muscleheads arrived at was insane. They thought warrior dood wanted to duel Ainz to show them Ainz's power so that they can find countermeasures? Really? Aren't the rampaging black monsters enough to explain that they don't and never will stand a chance? Isn't it better to find a way to survive under the demon god's rule, especially because said god appears merciful and reasonable throughout that farce of a fight?