Still can't say for sure that it's Regulus, but it's really an interesting thought -- that, at the end of things, Sirius' brother turned from the Dark Lord.

I still don't believe that Snape wanted to kill Dumbledore. He probably made the Unbreakable Vow with Narcissa because to do otherwise would be highly suspicious, and to give Bellatrix more reason to be suspicious of him would be stupid. Of course, he would have told Dumbledore immediately of this development, and Dumbledore would know about Draco's attempts to murder him and such.

I believe that Dumbledore was pleading with Snape to kill him so that Draco wouldn't be killed. As much as I'm sure Dumbledore would have liked to stay alive, he wouldn't want any of his students to die -- especially Malfoy, who had just demonstrated that he still had enough good in him to not want to kill him.

But that may just be hopeful Snape-worship. ^^;;;