I dont think you can avoid it.
Mehrune gets you to do it. it is like the only dialog possible:P

Nah, I waited a second and the cult guy approached me and begged me for his life and said he would just take the parts back to his 'museum'... me being the softy I am, let him waddle off back to Dawnstar... ofcourse Mehrune decided that this wasn't to be and two of his [censored]es teleported out and chased the cult guy, I killed them but a moment to late and my kindness was in vain.

Noooooooooooo! Go back with your saves man

The cult guy pleaded with me for his life too, but when Mehrunes says you gotta die then you die.

Ye and then you just fight the guy and kill him no?

You're generally much better off not even using the General Forum unless it's a REALLY basic question.

Wish I would have waited to get this quest instead of doing it at level 20. Anyone know what level to do this at to max it out so that it can be made Legendary? Best I can get mine to is 39 damage Flawless.