Oh, no, he did much worse:
He switched her brain with that of a chicken (with Magic!), then burned the chicken alive—in her body—so that the chicken’s thrashing around in horrible agony left marks around the room (while he forced Narcissa to watch, with Magic!). Then he kept Narcissa alive (with Magic!) in the chicken’s body, to keep company to Fawkes; he kept her hidden (with Magic!) right near his perch, so that every time Fawkes re-spawned she was reminded of what her captor was capable of. Then he burned her in front of Harry (with Magic!) just because he thought it was funny :-)
By the way, Lucius is not actually mad because he killed her—Dumbledore told the first part the above to him, but he kept silent about the details because it was embarrassing to have his wife turned to a chicken. That’s why Dumbledore had trouble during the trial; he felt a bit embarrassed about having killed Narcissa after all.
Also, Draco actually picked “fire” as his army’s symbol because he’s secretly fantasizing about being burned alive.
Maybe we’re overly pessimistic. It just occured to me that Aerhien’s world might actually be on the unlucky branch.
Suppose that the “branching point” is not whether or not the hero succeeds (that is, one branch lives, the other is destroyed). Instead, it could be whether or not the dust “finds” another way of attacking.
In the other branches, the defeat of the dust was finally successful (at the first try in one world, second try in another, or ~fiftieth attempt in the nearest-neighbor branch).
It is only in the unlucky branch that the dust keeps finding new ways of being nasty, and thus only in the unlucky branch there is a never-ending need to summon new heroes.
The other worlds don’t need to keep summoning heroes because they simply won, not because they simply lost.
Note that in this case most instances of Aerhien would live in in “nice” worlds. That is, among worlds with Aerhien, most measure is concentrated in successful worlds. However, among worlds with a newly-summoned Hero, most if not all measure is concentrated in threatened worlds (even if this is only a tiny sliver of the “total” measure).