This is the real point, I think. Everybody gets all "Dur! Daedra are evil! Hello Kitty is that way!" because it's mostly people who want good-guy choices complaining, but honestly, there are a lot of quests where you really have no choice in what is going on, or what minor choice you do have doesn't matter. The game itself might not be on rails, but most of the quests are.
THat's a valid complaint with a number of quests in Skyrim, but IMO, not this one.
First, as has been covered, you don't have to attack the Vigilant. He will attack you, but that's
his choice. You then do nothing but defend yourself.
Second, the person you're being sent to trap is himself
a priest of Boethiah - another blatantly evil daedric Prince. There is no doubt that this man is hmself evil. He's the one who desecrated the alter of Molag Bal in the first place by performing sacrifices to a different Daedra. It's not like he desecrated that alter by covering it with flowers and singing love songs.
To say you should join forces with this evil man so you can defeat Molag Bal is misguided - what do you think this man will do when you're done? Yup - more evil acts of human sacrifice and torture.
Third, although I was unaware of this - because I didn't follow the quest through - apparently you
can warn the priest - and he
then runs back to desecrate the alter with more evil acts. This is the man you insist on saving?
While you don't know that you're dealing with Daedra - and specifically Molag Bal - until you get in the house, you find out then. You, and a Vigilant of Stendarr - a person dedicated to destroying Daedra and their worship - have been trapped by this creature. You are stuck with no option but for one of you to kill the other. Why do people think that coming back with another Daedra's evil priest should somehow lead to a better result?
Yes, some quests should have different endings, or alternate paths. But for me, having a happy-happy-joy-joy ending possible for every single quest is dumb. Sometimes, you simply can't avoid having something bad happen. That makes a game better, IMO.
This is an evil quest for an evil creature. To me, it's fine that it has only an ending which some people find evil. It makes perfect sense.