I actually thought that Dumbledore wanted to show Harry that Draco, in the end, still could become an ally, which was why he stunned him. Now, (If Harry can think clearly), some of the things that Dumbledore said, such as Draco being manipulated to save his family, he might realize that Draco could help him against Voldemort... or he could just go on a rampage and kill everyone with Ron and Hermione.
But yea, it seemed that Dumbledore wanted Snape to kill him, because c'mon, how many of us believe that Dumbledore would EVER beg for his life... He just wanted Harry to reevaluate some people, and if Harry remembers the conversations, and EXACTLY what went on while he was stunned, it could make a difference.