This thread is likely to get massive, so I'd like to get a brief summary of my views on the first page. I'll try be terse, but since she's my favourite character and one that causes much controversy and conflict, it might be difficult.
-She's not inherently evil, I think that she is more or less amoral and narcissistic (clinically so IRL). I say this as I do not believe she actively seeks to make people suffer beyond what is necessary for victory or if she feels they have wronged her and therefore deserve reprisal. That being said, vengeance
is seen as evil in many cultures.
-One of the biggest claims for her being evil is her actions in the invasion of Gilneas:
1.) The decision to invade Gilneas was not Sylvanas', it was Garrosh's.
2.) Her use of the plague was the only logistically solid tactic the Forsaken had at their disposal. Garrosh demanded they conquered an entire nation and yet barred them from utilising the only feasible means of achieving victory. The Forsaken can't win a conventional fight. It was use the plague on an enemy they were forced to fight, or throw themselves into a slaughter.
3.) Resurrecting dead humans is not only the sole method Forsaken have at their disposal for bolstering their ranks and making up for losses, where other races can simply recruit and conscript, but it might also be considered a mercy in a universe where the afterlife is so menacingly ambiguous and seemingly controlled by capricious entities. The kindest and most noble human or orc will simply kill their enemies and leave them dead. The Forsaken put them down and then offer them a chance to get up again, albeit a little worse for wear.
-People also accuse her of conducting expansionist policy, which while true, to a degree, I do not think warrants accusing her of being wholly evil.
Let's look at the territories in which the Forsaken have been attempting, often successfully, to occupy and secure:
Hillsbrad Foothills, Silverpine Forest, Western Plaguelands, Arathi Highlands, Alterac and the Hinterlands.
The first 3 areas are indisputably part of Lordaeron and, with the majority of Forsaken (though not Sylvanas herself) being the group with the strongest claim to the blighted nation, there is little reason to describe the securing of these lands for the Forsaken as evil.
The Arathi Highlands are seemingly comprised of parts of Lordaeron (rightfully belonging to the Forsaken now) and the failed and collapsed city-state of Stromgarde, which for many years has been occupied by outlaws and murderous thieves, with no national affiliation. I see such a zone as essentially 'up for grabs'. The remaining citizens of Strom are either scattered and disorganised, with no conceivable government, or are now a part of the Forsaken, happily resigned to their new existence.
Alterac is another collapsed state, one which before the end of the Third War, betrayed the Alliance and the other human kingdoms. Another territory which is, in my opinion, 'up for grabs'. It is also an area inhabited by forces hostile to both factions with no national affiliation.
The Hinterlands is the only area which the Forsaken have moved into which gives me pause. But only very briefly. The Hinterlands as a whole do not belong to any recognised nation. There are forest troll settlements there, which neither faction recognise as sovereign, and some Wildhammer settlements. The Wildhammers have become members of the Alliance once again, so it is only natural for them to expect an imminent conflict with the stronger enemy force to the North. It's an open war, acknowledged by both sides, I see very little evil in moving into an essentially free-for-all zone and dealing with enemy settlements therein.
I think that many of the people who claim that Sylvanas or the Forsaken are evil are those who are upset by the fact that the Forsaken are one of the only faction-races achieving anything close to meaningful victories in an open war.
They're my main points. I'll happily respond to any retorts and likely add more points in defence of Sylvanas' amorality and, at most, passive evilness.