It's just that in the second chapter, when Bellatrix is being suspicious and Snape says he already knows what Malfoy is doing, he never actually tells them anything he couldn't have guessed. I think that all he says is that Malfoy will be rewarded very highly (something that Bellatrix says earlier anyway).
Now this could be taken to mean that he isn't as close as he would like to be to the Dark Lord and also he doesn't really know what he is promising to carry out (hence his hesitation with the vow).
So did he kill Dumbledore to get close to Voldemort for the greater good or for personal gain?
![:-?](./images/smilies/icon_confused.gif)