The DB aren't willy-nilly assassins. They only "officially" kill people who have been marked for death by the Black Sacrament, and to protect the Brotherhood. Killing because I failed a pickpocket... that's all on me. I'll still do it, but it's on me.
No. But contracts are repeatable as many times as you want, only sacraments are limited to one per day. So if you want to grind out the DB skill line, just go to the book, cycle through the contracts it offers till you get a murder spree contract, then go do it. Return, get another spree (those give more DB skill line points) and repeat.
If we are getting technical, all the contracts that appear in the book, do so because someone somewhere has completed the Black Sacrament. The rest of the discrepancy is all down to game design.