Anywhore xD? Wtf.
Well, regarding Sylvanas, look at her Warcraft 3 lore, if she had become a good hero, a truly good undead leader she would have been a very cool character, a bit like how they developed the orcs - they look evil but are just like any other race, they got good and bad guys amongst them. With Sylvanas, well, I am not much familiar with her WoW lore but in Warcraft 3 she was pretty evil, and while she was an elf she was just a basic bitch. So basically, being led by revenge doesn't make you good. Instead of caring for her people she is more like "how do I kill that motherfucker".
Oh and she started killing other races, being an undead now. Like a typical undead bad guy.
Also, regarding right and wrong, it's much like good and evil, it is subjective but there is a generally accepted objective view (like killing is wrong, normally, but killing Hitler is arguably wrong or right (or good or evil for that matter)). So when someone says "good" and "evil", it's technically like saying "everything generally accepted by the human kind as good/evil". Of course the world is made of grey, like, everybody knows that, but it's not only grey. It has black and white there as well. It's the whole spectrum. And there are just some things which CAN be described as evil: a psychopath raping children and then murdering them.
So lets go back to Sylvanas: Revenge? Arguable whether it's good or evil, but it's a rather basic desire. Being able to led go of revenge and do the more important things, like caring for your people and the new undead race, now that's something. And did she really have to use a poison arrow on Arthas to make him suffer? Sigh. Even more basic. Instead of just getting the job done and killing him. I think revenge can only be justified as long as you're not doing it for yourself but for the others. Killing Arthas would be a good deed to do, you would save many.
Lets look through what she did after she freed herself from the Lich King: Possessing other races with which you lived in peace before - now that's evil. I can't point out an undead character in Warcraft 3 who isn't neutral-evil/evil, even Kel'Thuzad started out as a neutral dude but ended up being evil.. and that's what I don't like about the undead race in War3. It's underdeveloped. Humans were obviously going to have both types of characters, orcs were amazing just because they had both types (and were much like humans), night elves kept a cool neutrality, unique to their race and very fitting (as they are forest people who have lived over 10 000 years and have wisdom at their side which leads to that balance they have - neither too evil, neither too good), demons were pure evil, as they should, but undeads.. eh.. Sure, they had more reasons to be evil, unlike the demons, but we never got to actually see a good character there. I was very disappointed that Sylvanas was not this character, as she seemed like she could have fit for that role perfectly. I feel like she is a wasted potential.