While an elf, Sylvanus actualy took a good role. Her bitchy attitude came from frustration that she could not defend her people, but she did stood last and accepted her fate to die for her people and lands. Pretty cliche heroic act lol.
Now, what WC3 role missed to mention is that she wasn't instantly made a ghost - Arthas actualy trapped her soul and tortured it for a very long time before she could escape. At that point she became neutral, unable to come back to her people and despising the Undead, but if there was a chance to stand back with her kind, she would. She couldn't choose, so she went and launched a whole new "race".