For as I said before, the last thing the Forgotten Realms needs (or Krynn, or Oerth) is a large number of insane and evil wizards and/or clerics running around looking for an artifact to corrupt so they can all create their own personal Anaurochs.
The Forgotten Realms and other worlds need Champions to stand up against this kind of threat.
I still say Mystra is neutral. If Vile Magic exists, then I cannot believe Mystra is Good.
For Mystra is the conscious aspect of magic, and if there is Vile Magic, that consciousness would not be good. Not, at least, in my opinion.
I almost feel like I'm putting my foot in my mouth by speaking here, but a few key points:
Evil Wizards already HAVE done this in the FR setting, more than once. See Myth Drannor, Hellgate Keep, and the Anauroch which was caused by Phaerimm, who are natural evil wizards. They currently ARE doing this as well, and have never quit looking for evil artifacts to corrupt to use toward these ends.
The Chosen of Mystra also are not in place to stop this type of evil specifically. They exist as Chosen to prevent the abusive use of magic. Although the Chosen have become politically motivated on the basis of individual beliefs, the group as it has been stated to exist is to stop others like Karsus, who endangered the future of all magic.
Next up, I'm not going to dig it up at the moment, but the logic behind Mystra still allowing evil spellcasters to have access to magic has been stated before in many places. Vile magic is magic, and as long as it doesn't endanger magic as a whole, it is allowed by Mystra. Evil spells fall into the same catagory.
The only type of magic Mystra has ever limited or cared to limit was 12th+ level spells in older editions. (Mystra has no direct ability to limit Shadow Weave magic, although her followers attempt to limit followers of the Shadow Weave).
She doesn't care about the level of destructive magic unleashed in the realms. While it certainly may be more logical that she is neutral, Greenwood has had funky logic for wanting her the way she is for some time, and has consistently held to that logic.
Then again, if you'll take a character to 121 level using a homebrew exp system that encourages such, why not just make her neutral, be done with it. End of story, no debate needed, it's a house rule just like the levelling system.