My noble warrior wants to Destroy the Dark Brotherhood, but how can I justify doing the first DB quest? I know you could say, "She deserved it", but a good guy wouldn't just kill someone because 'they deserved it'. Furthermore, how could I justify pretending to be an assassin in the first place? I think it's annoying that you can't skip that quest and just jump to the destruction of the Brotherhood.

Remember, it's not the DB that want her killed, it's one of the children that she has abused. He is just asking the DB to do this deed.

Yeah, but you're still murdering a person on the kid's behalf. Maybe she does deserve it but like the OP said, a good guy wouldn't just run up and murder someone because they deserve it. At worst she's a child-abuse, and as bad as that is, she ain't Hitler.

Imagine she did worse to him...would justify her murder in my mind.