Desmond and Sam were gone. Drake had nearly fulfilled his end of the deal. All that remained was to kill Sam on their way back, and the mission was complete.
He had no idea why Stephanie wanted her dead and why she had to go through such a long process of doing so when she could have easily been dealt with by one of his hitmen.
From what he worked out with Stephanie, he would make it seem like he would let Sam go, only to have her killed after Desmond puts up a struggle and tries to save both of them. He would only kill Sam and just injure Desmond a little, then have her come with the police and save him.
Drake didn't want any of his men captured, so he made an escape route for them so they could disappear as soon as they were done with their act. As Drake laughed at the stupidity of it all, he took a good look at his minions counting the large stash of money from the hellishly large briefcase Desmond brought with him.
