There are a lot of villains in Warcraft. They have different Modus Operandi, and motivations (When they have motivation) but there is no denying there is a broad spectrum of evil doers out there.
Illidan – The recklessness Illidan displays makes him a danger to himself and others. He goes full tilt on every problem he encounters even when it’s not necessary, and by doing so, he does a lot of collateral damage, which he then seems largely indifferent to. A less obvious villain trait that Illidan displays is his dishonesty, not that he outright lies very much, but he does make a point of withholding information from his allies. No one knows what he is planning or what he will do next. You know Illidan is never going to have your back, because he is to preoccupied with his own plans to consider the consequences for anyone else.
Gul’dan – For all the lip service orcs pay to Honor, their culture is really build on the concept ‘Might Makes Right”. They do not use logic or mediators to resolve conflict they use violence. The traditions and trappings of honor they uphold, are simply a way to hold combatants to only using their raw physical prowess in battle. It is intentionally set-up so that you can not trick, fight dirty, or outsmart your opponent. Whoever is physically strongest is correct in all matters of debate.
This set-up is a real problem for a orc like Gul’dan who is clever, but not physically very strong. Gul’dan sees though the veil of honor that orc praise, and knows that at the heart of things, power is what they really respect. Honor is discardable because they live in a society where killing your opponents is a legitimate form of conflict resolution.
Once Gul’dan realizes his potential and rules over orcs though the Shadow Council, he becomes vindictive. He’s bitter and resentful of orc society for how he was treated when he was weak and revels in being about to lord his power over those that would have done the same to him.
Arthas – Arthas is like the inverse of Illidan. Whereas Illidan goes out and jumps the gun trying to handle problems, Arthas was reactionary. He followed a problem, to it’s source and by the time he did something about it, things were desperate. The thing that makes Arthas so effective as a villain is that, even though you may not agree with what he’s doing, you understand why he’s doing it and why he thinks it’s the right thing to do. That understanding of why he’s doing makes it possible to imagine that in the same situations, you might feel like you have to do the same things.
A good villain should be relatable to the player, and have a reason to give a hero pause. Make them wonder if fighting them is the right thing, even for just a moment. In addition to being a difficult physical fight to challenge the heroes attempt to stop them. They should also offer a narrative reason to challenge the heroes morality or conviction in their ideals. Even the most evil and malicious villain is more effective when we can see something of ourselves in how they act.