The thing is, I met a talking dog, and not being prejudiced I listened to his plight and agreed to help him out. How was I meant to know that I was sending my ultra good-guy character on one of the infamous "Daedric Quests"? As I followed my canine companion along the road two armoured fighters rushed out of the woods and I instinctively moved to defend my new friend. Only when Ria lay dead at my feet did I realise in horror that the attackers were Companions, who I also happened to lead. Well, you can imagine how the rest went down. I was forced to kill the others due to their sudden (but not so mysterious) animosity towards me. Now my good-guy is tainted forever, and he has to live with the ghosts of his former friends on his conscience. You can also add a few Vigilants of Stendarr to my bodycount too since they now attack me on sight, and I weep as I hew them down.
The thing is, how was I meant to know I was taking a Daedric quest? I avoid all spoilers for games like Skyrim and on the one hand I think "okay that's fair enough, I guess evil doesn't always announce itself", but on the other I think, "that's annoying". My Oblivion obsessed friend assures me that this is perfectly in line with the Daedric Prince in question (I never got really into the lore myself) and so again, I think it's kind of okay. I almost like my Anakin-esque fall from grace. Is there any way I can earn redemption though? It doesn't matter so much in terms of the Companions, since they're dead, but I hate to be marked as an "Evil One" for trying to help a dog out on a rainy night.
EDIT: Oh yeah and some of the Companions took considerable murdering. They kept going into that "prone" stance until I eventually triggered a fatality on them. It's the only way to be sure.