As it's been said already, good and evil are subjective. What's good to someone might be evil to someone else.
Case in point, my Kaldorei DK.
Saev was a former Cenarion Guard (Warrior). Never married, had no kids, was your typical Elune-worshipping, honorable Kaldorei. Was shipped out to aid in the assault on Icecrown with a whole group of his fellow Guards. His battalion was sent to Jotunheim to assist in establishing a foothold there (base camp, if you will, from which to expand operations), and subsequently buried in an avalanche. The ones who weren't buried, were picked off by Vrykul trying to get back to the incoming reinforcements. Rather than put some to defense and try to recover the lost, Saev and his brethren were left to die. Succumbing to frostbite, hypothermia, and eventually suffocation, he was raised as an initiate and served Arthas. Insert time as being a truly evil harbinger of death here.
He gets out, still holds a grudge at the fact that they were left to rot. While not exactly angry, he's not happy with it either, but what can you do? Past is the past. He picks up Inscription to improve the coordination in his frozen hands and fingers, sets himself to the mentality that while most living people hate his class and everything it stands for, they're capable of being just as heartless and cynical as he is. He's not good, he's not evil. He's indifferent. People will die, wars will happen, he will have to kill people, and people will be out to kill him. It's the way of the world. He's cynical, sarcastic, and pretty much a bastard to everyone on first encounter, because he has no tolerance for idiots and goodie-two-shoes people. He knows he's killed people. He remembers it pretty vividly, but if you're going to sit there and condemn him for it, you better have never slain a single creature out of anything but self-defense in your entire existence, because he's going to throw it right back at you.
To many people, he seems like an evil, cold-hearted bastard. To a handful who know why, he's simply making the best of the situation fate's put him in, and isn't evil at all. He's not going out to defend the honor of every hapless maiden he crosses, or saving kittens, or rescuing kids from the SW canal (though he would if he came across it), but he's not going around randomly slaughtering people either. He tries to adhere to the same code of ethics he had in life, if only a little bit more sarcastic and lax at times.
So yes, being "good-aligned", while subjective, is perfectly viable for any class that plays this game. Just as being "evil-aligned" is obviously the same. Someone made a reference once to the Scarlet Crusade. Evil to us, but to them, -we're- the evil ones. It's all in how you look at it.