Board Thread:Lore Discussion/@comment-6716700-20130319133042/@comment-3444868-20130320233421

No one actually likes the Dark Brotherhood until they want or need them for something. Every faction is hostile to them. The Morag Tong was the only assassin guild that was legally accepted.

Also the Brotherhood maintains neutrality (as much as it can) so it won't be singled out and destroyed and/or lose a group of clients. Because if there is one thing they love as much as murder, it's money.