Well we're a bit off topic... but I guess it always comes down to the big unsolved issues, since everything is interrelated.
I'm pretty sure that argument that Hagrid overheard between Snape and DD in HBP was about following through on the Unbreakable vow... Remember that, although the vow was about Draco, we are told that if you don't follow through on the UV, you die... So in theory, yes Snape was preventing his OWN death...
Although... I seem to recall the source of information about the UV was Ron, which was what he was told as a kid when Fred and George tried to get him to take such a vow. It's quite possible this was something that Arthur told his son to insure that he never try one or take one...
Suddenly it occurs to me that maybe the truth of the Unbreakable vow is that you are magically compelled to follow through, regardless of your own desires (hence the look of revulsion)... And DD's plea was that he not fight it, since it could blow his cover... And to that WWW, I really hadn't thought it through to that point before, as I was still erroneously convinced DD was alive, but if this helps you think Snape's not a death eater, all the better :)
This would also fit with Snape protecting Harry throughout the series as a result of my other theory that Snape took an unbreakable vow with DD to protect Harry (the reason DD trusts Snape implicitly???) since his instinct is to find every way to annoy Harry, his school rival's (James Potter's) son...
And no matter what, I can't imagine DD would do anything like a Horcrux or become a ghost... In PS/SS... he said, on the subject of Nicholas Flamel... that death is not something to be feared to "the well-ordered mind"... Dumbledore's portrait in the office is the best we can expect, I'm afraid, and I have a feeling that his ability to provide input and advice will be especially unsatisfying to Harry, in particular... On the other hand... at least he'll be able to walk out of his portrait and watch every kid in Hogwarts who has his Chocolate Frog cards.