You'd been turned by the Volturi a long time ago. But it takes you more than eternity to really get used to the ways of the undead. They are... unique.
Every time you think you've got them figured out, there's an entirely new Volturi habit that's beyond anything you've seen.
Aro has been quite impressed with you. Your gift is unlike anything he's ever seen before, you fascinate him. He's incredibly proud to have you as his new apprentice, and it's safe to say he's always treated you specially.
So for your 100th birthday, he gets something special. Even though you've been undead for at least seventy years, you're still used to the human's norms and values. So the vampires always have their way of surprising you.
Aro finds you in your chamber, his hands behind his back. "Happy birthday, my dear. I've got something for you."
Aro reaches his hand out, holding... a head by its hair. You recognize the severed head as someone who used to bother you a lot. Apparently Aro has done you a favour...