Why? It should be on Bethesda to make sure all quests can be completed by any character, regardless of how they choose to play the game. Not us. They're the developers and Skyrim is an RPG. It should cater to role-players by giving them the options they want rather than features they have to ignore.
More options would be good, of course, but expecting EVERY POSSIBLE character personality to be able to have an appropriate solution to EVERY quest is highly unrealistic. Even if they make a "goody" and "baddie" resolution to every quest, any reasonably well realized character is still going to have problems finishing them all. Lawful Stupid and Stupid Evil aren't the only two psychological types out there.
The best approach is to do what they have done: provide one or more options to proceed, and always provide the option to walk away (i.e. not act out of character). If your character would not do a quest, then for god's sake, have a little role-playing courage and freaking
walk away.
As for being locked in a room with an innocent man who is homicidally possessed, you actually still have a choice: defend yourself like any reasonable person would, or (if your character really is a complete saint) stand still and let him kill you rather than compromise your values. That is a fantastic ending to a story about the nicest guy in the world, but if you want to be that nice, you have to pay the price. How committed are you to that character's purity of action?
What would that character actually do in that situation?Obviously your character doesn't want to kill the guy ...
that's the point. You walked right into Molag Bal's trap (the priest even warned you about how the place was evil - what did you expect?) and now you have to decide whether you're willing to kill to get out. If not, it's game over for the Dovahkiin, and congratulations on not letting the Prince corrupt you. Maybe your next character will be a little less stupid and/or more flexible.
In good fiction, though, the hero would defend himself, and agonize forever about what he'd been forced to do. This is called
character development and it's a good thing. Having to make a choice that breaks your character's code only makes him more interesting and rewarding to role-play, not less.
The only way Bethesda screwed up is in not allowing us to drop or sort quests in our log. Personally, I wish I had a folder where I could file "Quests I Don't Want to Do", basically like a Windows Recycle Bin. So they would still be available if I changed my mind later, but they wouldn't clutter up the interface the rest of the time.