I guess the problem is that there isnt really a moment that we can point to where she definitively realizes that what she is doing is evil.
We could of course assume that she started having doubts once she actually went to the Maw and saw the souls there and realized the true extent of her sheer villainy, but it still doesnt gel with her characterization in BtS or BfA.
Remember, this is a person that killed her own people because they were not truly forsaken by finding happiness reuniting with their loved ones. She raised Derek as a slave, violating her own stated principles on the importance of free will to make him an assassin meant for killing his own family. Raised Night Elves that she herself killed to kill more of their own.
The problem is that Blizzard made her too evil in BfA. Her acts of villainy is on par with penultimate evils like Sargeras.
Maybe the WoW: What's next panel will shed some light though. One can hope I guess.