How about DD pleading for his life. Do you see that happenning? Someone who is not afraid of death, who jumps in front of the enemy just to ask him nicely "please don't kill me"?
I think he sacrificed his life first for Draco. If Snape had died for the unbreakeble vow, LV would have punished Draco. Then DD knew he was pretty weak and maybe he thought Snape as a spy would be more important for the future war. He just chose the best way for things to happen and tried to save as many lives as he could. And maybe he was going to die anyway from the potion he took.
And I saw somewhere on the forum another argument, some time ago, I'm just going to remind it: It was about repetition in the book. So many times we thought Snape was evil, and we were always wrong to blame him, In the end he proved to be on the good side. And another repetiton was the hate on Snape's face killing DD, just like the one on Harry's face feeding him the potion. (Sorry can't find the topic right now so I don't know who actually said this before.)
Then maybe JKR is just plannig not to repeat things but turn around dramatically.