Remember, if you kill Astrid to start this quest, you can never join the Dark Brotherhood. Kill Astrid (and if you want to be crazy, the hostages too. They might be in on it, you never know). Loot her body for some good gear, then head to Dragons Bridge outside of Solitude. Talk to Commander Maro who will send you to kill the Assassins in their sanctuary. Kill them all and return to him for a reward.