It was a "please kill me" sort of plea, I think. Snape, however, was also bound by the Unbreakable Vow, which pretty much required him to kill DD... An excellent plot twist.
Rowling really did a great job on this one, I think.... The only question is: does Harry go back to Hogwarts for Year 7, or is she going to compress several years into the next book?
Yes, an interesting question. Will it end decades from now with the Hogwart's head master - the elderly Harry Potter - reminiscing about his glory days?