HARRY POTTER & THE HALF BLOOD PRINCE QUESTION?

Book or movie, Snape was originally with the "bad guys". He was Voldemort's man. Then Voldemort killed the woman he loved (Harry's mom) and he turned double agent, reporting to Dumbledore and the good guys while making Voldemort and his followers think he was still a bad guy. I doubt that Snape was Voldemort's second-in-command; Voldemort planned to live forever and didn't want a successor who might get impatient waiting for power. Snape kills Dumbledore, which makes Voldemort, Harry, and everyone else certain that he is evil, but he and Dumbledore knew that A) Dumbledore was already dying slowly from the curse on one of Voldemort's horcruxes and B) Voldemort had told Draco to kill Dumbledore, and if he did it would destroy Draco. So Dumbledore forced Snape to swear to kill him before Draco could. Hope that clears it up some; it is a very confusing plot if you haven't read all the books.