that doesn't help improve game play experience at all, learn to discuss.
1. I think you can choose not to enter the house.
2. I know you can choose to decline the rest of the quest
I think it is awesome design when you get a quest like this although on my RP character I skip all the dark or even grey quests.
As for what happens inside.
It's just life.. You got dragged in a really "shi..ty" situation. There is no sugar coating it.
You had to make the most humanizing decision there is: survive no matter what.
In a normal situation you would never kill that person, but this was not a normal situation.
Deal with it. Grieve, have nightmares, go to shrines, give away your riches, dedicate yourself to purging vampires, necros and all the other evil. Live the game.
And go do the dawnstar nightmare quest. It will make you feel better.