Chiming in as an Athiest. I quit my character rather than complete this questline. I'm not even comfortable playing it with my assassin character. I find it one of the more disturbing quests I've seen in gaming. It's just so "ho hum" about killing people. Even the Dark Brotherhood seems to have an ethics about assassination and a reason for doing it. The quest you're talking about is pretty poorly presented.
....I'm almost speechless.
You
stopped playing a character, because you couldn't find it in yourself to refuse to continue in a quest/situation that became objectionable? That's so bizarre to me.
Yes, it'd be nice if they had a couple more lines of dialogue, where you could report the problem to the guard captain, or say "Go to Oblivion!" to the Evil Voice?, but..... seriously, think about it. From the point of view of the character, any good adventurer could fall into this situation. Guy asks you for help. You go into the Evil House to help him. Things spin wildly out of control. What does the good character do? He leaves and refuses to serve the Evil Voice! Why wouldn't he?
Just because the game presents you with a situation, doesn't mean you have to go forward through it. Especially on an optional sidequest. Just because some words are in your "tasks you could do" list (i.e, quest journal) doesn't meant that something is FORCING you to do them! The idea of throwing away a character, and all the time you've put into them, over something so phenomenally trivial just completely blows my mind.
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A different way to respond to the OP occurred to me before I'd seen this post:
Why did I just beat an old man to death with a rusty mace?
Because you
CHOSE to.