Just a few thoughts about my interpretation of the scene on the tower. Best to steer clear if you haven't finished the book, or if you have and are now convinced Snape is an irredeemable monster.
Funny the sorts of things one notices in moments of extremity.
Draco's face is pale and his skin looks clammy; there's a bead of sweat rolling lazily down his jawline and his hair is in a state he would have considered too deplorable for public display a couple years ago. The tip of his wand is not quite aimed straight at his headmaster; it jerked up a bit when the other Death Eaters arrived to join him here atop the tower, but it's still tilted just barely too far downward and it keeps wavering a little; the whole effect together puts Snape in mind, bizarrely, of a wilted flower.
Dumbledore is equally pale, disheveled, ghostly. The kind of pallor that cannot happen in a day, but adds up slowly over weeks, months, years. Snape can see, because he knows how to look for it, the way his headmaster's hands are trembling; the burned wand arm more obviously, but both of them even so.
Senses honed from years spent in the dungeons notice a faint scent of Wit-Sharpening Potion from one of the Death Eaters behind him. Not the werewolf, he's guessing.
Someone cast Petrificus Totalus up here a few minutes ago.
He feels like he's stalling for time, indulging these thoughts, though he knows only seconds have passed. But he doubts he'll get more than seconds to make this decision. The people behind him aren't really inclined to wait politely at the moment.
Draco's hand steadies itself, gradually.
And Dumbledore's eyes are pleading with him, and Snape has known for a year that Albus Dumbledore is already dying and he is going to have to deliver the finishing blow and it's not fair and it's not right and isn't it just a perfectly fucking typical way for everything to go.
Draco's elbow locks in place, his eyes steady on some spot just below Dumbledore's eyes as he steels himself to walk down a path he can't return from.
Snape knew it would come to this. Hell, he helped it along. He has no right to feel, even for the briefest second, as if he is about to kill his father to save his son.
But he does anyway. Only, of course, for the barest moment.
"Severus, please," Dumbledore whispers, and Snape thinks this may be the first time in their long acquaintance that Albus Dumbledore has asked Severus Snape to do something, not to further the cause of stopping Lord Voldemort, but... simply for Albus Dumbledore.
Not that this will all go to waste, in the grand scheme of things.
The old bastard is shrewd like that.
The only thing Severus really regrets, as he shouts the curse, is that he won't get to come to the funeral.