That's a weird thing to do if you're against murder for hire. The quest starts from the premise 'No, I actually don't want to murder for hire, and in fact I think all of the assassins who do should be killed instead.' However, to start that quest, the first thing you have to do for that quest is accept a small child's contract to murder an adult, while pretending to be a member of the Dark Brotherhood.
Destroy the Dark Brotherhood is a weird quest.
