Both "evil Snape" and "good Snape" are too pat for me. I think Snape used to be loyal to Dumbledore up until the beginning of book 6, when he switched loyalties to the Malfoys. Snape's motivations seem to go beyond loyalty to either side; what he wants is appreciation and respect for his powers. He may have appreciated Dumbledore giving him a second chance and saving him from jail, but if a beautiful woman came up to his doorstep and begged and cried and pleaded for help, this would stroke Snape's ego enormously, and compel him to do what she wished.
I don't think Snape is Dumbledore's man through and through. Snape is too intelligent to have been cornered into taking the vow, and if he wanted to remain a plausible double agent, all he could have said was, "Sorry, can't take the vow; that would be defying the Dark Lord". I can't see how Snape would have told Dumbledore afterwards ("Hey, Dumbledore, I took a vow to kill you or die, but we're still cool, right?"). Finally, I can't see Dumbledore, who's supposed to be very noble and above Voldemort's methods, ordering a colleague to perform murder, using dark magic.
I also think Snape's motivation might be that he pined for Harry's mom, switched sides to protect her, and now wants vengeance for her murder. Killing Dumbledore would actually help his plan, since it earns him Voldemort's trust, bringing him closer to Voldemort where he can attempt a final blow. This would explain why Snape was so remorseful about leaking the prophecy to Voldemort, why Dumbledore believed Snape, why Snape curiously never taunts Harry about his mom (It's always James), why Voldemort offered Lily a chance to live (could have been a favor to Snape), and why a jilted Snape could be so easily manipulated by a woman like Narcissa.
