Well, seeing everything you love and care for be destroyed, then you are killed, resurrected in a horrific way, turned into a puppet unable to resist your master’s commands, forced to do horrific stuff against your own people and innocents, then finally freeing yourself and spending years hellbent on vengeance, only to find yourself empty of purpose once your target is dead, so you take the suicide route but you can’t die because a horrific afterlife is waiting for you… well, that changes people. A lot.
To make matters worse, she’s undead. Being undead do not feel the same as being alive. The sensations are all different, your needs are different, and Sylvanas has no reason to have any hope or empathy for anyone else. For her, existing is painful, life has no purpose, everyone around her is going to die anyway. Her unlife is torment, so why should she care if anyone around her suffers? Nothing will ever compare to her own suffering.
Sylvanas is a person broken beyond reckognition. She became what she despised the most, and is now a dangerous psychopath that has no regards for life. All she cares about is one thing: herself. Anything that threatens her existance is a menace that must be stopped in any way available. Everyone around her is either a pawn or an enemy.