Harry Potter holds a number of grudges against former professor Severus Snape.
He remembers his occlumency lessons, those countless "Empty your mind, Potter!", only to have legilimens cast on him a couple of seconds afterwards without further preparation or instruction.
He remembers his potions lessons, all those times when the professor vanished a perfectly acceptable potion from his cauldron to mess up with his grades and his volatile temper.
But today is payback day.
The final confrontation between both has been a ruthless display of the dirtiest underhanded magical combat techniques and magic mind battling. In the end Harry has gained the upper hand and professor Snape stands, battered and disarmed, at his mercy.
"So in the end it seems there was something to say for fame and silly wand waving, wasn't it, professor?"
"Stop your useless gloating already," pants Snape, "you are a boastful moron like you father." His greasy fringe obscures his downcast face, but his eyes glare malevolently from behind.
"Come on, professor, don't be so bitter. I just wanted to tell you that not everything you taught me was useless. In fact I learned quite some interesting magics from you."
"What the hell are you talking about, Potter."
"Ah, professor, such fond memories we share, don't we? Of all the things you ever taught me... for instance, remember our occlumency lessons? Let's do it once more, just for the old times sake, eh?" His voice turns suddenly serious and points his wand at Snape's head. "Empty your mind, professor," says Harry with an evil glint in his eye.
Snape just raises his head defiantly and observes Harry with a confused look.
"Come on, Snivellus," insists Harry, "be a good sport. Empty your mind, let go of all emotion!"
The potions master stands silently pouring all the hate he is able in his piercing gaze.
"No? Well, if you refuse to empty your mind I guess I will have to do it for you then, won't I?"
A look of comprehension and utter horror appears in Snape's eyes, but only for one moment.
"Evanesco," hisses Harry, and he gracefully flicks the wand pointed to the death eather's head.
Snape's skull produces the strangest hollow sound when it hits the stone floor as his owner falls dead.