But what is the point, Dumbledore had more chance to help Harry against Voldemort the Snape, instead of losing a spy and keeping the the leader of the OOTP they lost both. What exactly would be gained from getting Voldemort's trust, if anything he'd either kill or punish Snape for doing Draco's job. How can Snape help Harry from killing Dumbledore? Only Harry knows how to kill Voldemort, unless Snape was told about the horcruxes which...okay new theory (I know it contradicts my original point but just throwing it out there)
I will go so far as to say there is a possibility that Snape isn't evil and that he's trying to gain Voldemort's trust so he can find the remaining horcruxes for Harry, but it is very unlikely since Voldemort has nothing to gain from revealing the hiding spots and is very unlikely to tell anyone about them.