Well really I think the Horrific Visions handled it pretty well. She ultimately succumbs to the Void, but she does so in good faith. She believes that the Void is simply the only way, that everything else in a lie, yet I don't catch any pleasure or satisfaction in her words. I think she just realizes that's the only way and there is no point in resisting anymore. She's not someone like Kael'thas who actually revels in that madness. And indeed when she dies she realizes what she's done, she sees the light again.
I think Alleria could be a very good tragic villain, someone whose desire to protect others ultimately led them to their own downfall. As they say, the path to Hell is paved with good intentions. I'm just kind of skeptical that those hacks at Blizzard can handle such a character well.