If you want to get a reward for helping Ainethach or Atar, you can talk to them, intimidate them, and report back to your ally. Atar can be contacted first, or you can intimidate Ainethach to kill them. You will earn septims for completing the quest, no matter which way you choose. For the best rewards, it is best to support your ally.
How to Get a Reward For Helping Ainethach or Atar