I see that we either have quite a few extremely disturbed individuals in these forums, or quite a large number of them are giving "ironic" answers to this poll.
The Dark Brotherhood was quite clearly invented just to give players the option to be as completely evil as the game would let them (I mean, in Oblivion, they decorate with dead babies), and everything they do is
definitionally evil.
In fact, let me just quote from the wiki some things, which are obvious spoilers from the Oblivion DB questline:
Spoiler http://www.uesp.net/wiki/Oblivion:The_Night_Mother
After slaying Bellamont, The Dark Mother will approach you and initiate conversation "So, at last we meet. I have been following your strange journey through the Dark Brotherhood, young one. Your killing of the old man, Baenlin... the execution of Adamus Phillida... the way you stalked and murdered each member of the Draconis family... Your Purification of the Cheydinhal Sanctuary. You, a mere underling, even managed to single-handedly eliminate half of my Black Hand!".
You then have three options to respond.
One is to say nothing, which says nothing about your character.
One is to say you did it all in service to the Black Hand, which in turn means Sithis, which is a spirit of pure destruction. That means your character motivation is that you literally want to destroy all of creation. In short, a total nihilistic omnicidal maniac.
The third, however, is probably most revealing:
"Ha! I reveled in their slaughter!"
"So you did, my child. So you did! Ha ha ha ha ha! You obeyed your orders and showed no mercy or remorse. How could I ask for a better servant? If only my other children had served me so well! You see, I have known of Mathieu Bellamont's intentions since he was just a boy. I knew of his thirst for vengeance. I could have informed my Listener. But I refused! Refused to reward such incompetence! Ungolim was weak. Indeed, I allowed Mathieu Bellamont to proceed on his destructive course. Just as I allowed you to intercept him."
Or, in other words, it makes
absolutely no difference who you kill to your character when you are in the Dark Brotherhood. You could literally have been sent off by Sheogorath to murder everyone simply for having a name starting with the letter "D" for all your character knew. They could have just pulled names out of a hat to kill completely at random. Didn't matter, your character obviously didn't care, they just kill for either money or the joy of inflicting pain and suffering on others.