Of course you never said that. Because she wasn't when she was in your backstory. That's the stuff that happened afterwards.
Okay, I'll put it more precisely. The player never said anything about her mom having the Intelligence score, native talent, or inclination to become a powerful wizard. And she never said anything about her mom having the mental instability to become a homicidal lunatic. And she certainly never said anything about her mother having the slightest desire to become undead.
So unless these traits were somehow imposed on the mother from outside, you're adding HUGE new elements to the player's backstory and radically rewriting the concept she had. In fact, even if they
were imposed from outside, you're still radically rewriting the concept, you're just adding a feeble in-game excuse.
I'm curious if there's a correllation between people who think this is the GM stepping on the PC's territory and those who earlier in the thread were talking about the opposite; it "sitting wrong" with them to have players treading into GM territory.
Can't speak for anyone else, but I agree with both. I have problems with the DM invading player territory, and I have problems with the player invading DM territory. Character background is part of the concept that defines a PC in the game world, and therefore I consider it firmly in player territory*. The DM can certainly
use the NPCs introduced in the background, but if s/he does so, I expect those NPCs to be used as presented, not totally rewritten.
If you need a villain and none of the NPCs in my backstory is suitable, make your own. If you need a villain with a past connection to a PC, ask me to come up with one.
[size=-2]*Caveat: While character background is player territory, introducing elements into your background that affect the makeup of the campaign world is infringing on DM territory.[/size]
To me, this is clearly GM territory. I mean, it's about as clear as can be. It's a setting element, pure and simple.
Then why are you asking players to create it? Why didn't you just inform the player that her mom was a murderous lich?