Severus Snape kills Dumbledore using ''Avada Kedavra,'' a you-gotta-mean-it-for-it-to-work Unforgivable Curse.
Once a Voldemort-lovin', half-blood-hatin' Death Eater, always a Voldemort-lovin', half-blood-hatin' Death Eater.
He's a master of Occlumency, the ''magical defense of the mind against external penetration.'' Few can see through his lies.
Despite ''trusting'' Snape, Dumbledore refrains from appointing his former pupil Professor of Defense Against the Dark Arts for years. The second he gets the job? Death all around.
Dumbledore admits that being ''rather cleverer than most men,'' his mistakes are ''correspondingly huger.'' Trusting a man who's betrayed him before? Huge mistake.
Harry's been saying it all along.
Snape must fulfill his do-or-die Unbreakable Vow to protect Dumbledore's would-be assassin, Draco Malfoy preserving his cover as a double agent. So he reluctantly kills the Hogwarts headmaster only to continue spying on Voldemort for the Order of the Phoenix.
Dumbledore was ordering Snape to kill him, not begging for his life. He'd never beg!
Even while battling Harry, Snape reminds him to ''keep your mouth shut and your mind closed'' sound advice for his former Occlumency pupil. Plus, Snape has rescued Harry several times however begrudgingly and even tells the Death Eaters in book 6 to hold off on killing him, supposedly under Voldemort's ''orders.''
Dumbledore trusts him ''completely.'' That's good enough for us.