No one can be pure good. Even if you play as a High Elf, in
The Elder Scrolls they're no better than humans. And Molag Bal is a god. Sure, you can ignore him. For a time. But eventually he's going to grow impatient. That's what's going to happen to my character. He tries to ignore Molag Bal, but eventually realises that you can't cheat a god. It's going to haunt him for the rest of his days, but it's also going to make him a better person. It is going to inspire him to do more good, and fight harder to save Tamriel.
So a Daedra God made a dude crazy enough to try to kill you and when 2 minutes later the same Daedra God offered you a prize in his evil voice you was like "sure, gimme that prize!"... I'd hate to break your bubbles but if you roleplay a good character then you should have simply walked out of that house without checking out what the prize was. YOU chose to accept an evil prize, YOU decided to become evil, noone forced that choice on you.
How can you
walk out of a trap set by a Daedra. Besides, the house was barred until you accepted.
This was sort of my experience as well. Molag Bal told me to crush him. I look over at the guy for a few seconds and he does absolutely nothing. Since I always like daedra quests I obviously smacked his head in a few seconds later.. So to me it appeared I had the choice of just running out of this building without killing him.
Then you must have stumbled upon some kind of glitch, because in my version he started rambling, then attacked me.