В lj-сообществе по Плоскому миру приводится замечательно простой аргумент в пользу того, что Снейп на стороне Дамблдора:
Why Snape isn't evil? (на самом деле, конечно, "почему Снейп не предавал Дамблдора", т.к. он evil по определению, в независимости от...)))
Consider this: what does Snape say when he kills Dumbledore?
Answer: nothing.
Snape just looks at him, kills him and leaves.
Now remember what Vimes says in "Men at Arms":
"Something Vimes had learned as a young guard drifted up from memory. If you have to look along the shaft of an arrow from the wrong end, if a man has you entirely at his mercy, then hope like hell that man is an evil man. Because the evil like power, power over people, and they want to see you in fear. The want you to know you're going to die. So they'll talk. They'll gloat. They'll watch you squirm. They will put off the moment of murder like another man will put off a good cigar.
So hope like hell your captor is an evil man. A good man will kill you with hardly a word."
Snape never gloated. Not so much as a "You old fool." He killed Dumbledore with hardly a word. (с)
перевод
Хотя, надо заметить, в других случаях он показывал себя весьма power-addicted и не упускал случая вволю поиздеваться над теми, кто находится в его власти (читай: студентами). Но не над Дамблдором.