So there is no option to actually tell the priest about the plot when you are sent to get him?
You can tell him afterwards you used him as a bait. He will be happy that the cannibals are dead and will reward you with a leveled amount of gold (around 1000-1500 pieces)
Oddly enough attacking Eola will be counted as an assault, even it's a proper quest choice (Eola is set to essential before that). To avoid the assault charge (out of place imho) I used a frenzy poison and watched the cannibals battle between themselves. Eola is strong enough to take away all of them, even the two necromancer priests (odd choice for a necromancer to become cannibal... first they eat you, then they resurrect what's left?

). using a frenzy poison will also guarantee that, back in Markath, the relatives of the poor, deceased cannibals, won't be hostile towards you (ie the shopkeeper).
So far I completed most of the Daedric quests, except for three in which I went against the will of the Daedra: Namira's, Vermina and Molag Bal. I did Mephala's quest only to help the good Jarl of Whiterun but then I sealed the blade away in the Labyrinthian dragon masks shrine. Boethia's quest was borderline but, at least, she has a point: being selfish is entirely a matter of personal gain and ambition.
I don't mind being a little evil for my own gain but I hate when the only choice is "I am evil because I like to be mean and to hurt puppies". In that case, I'd rather turn down the offer or work against the ones that put me through useless ordeals.