I can't decide which is a worse justification: "It was in the comics" or the more popular "she didn't go evil offscreen, it was set up in the tag scene of WandaVision."
Waldron has done both in his interviews, saying that Wanda goes nuts in the comics and that it was a logical jump from the tag scene of WandaVision to the movie.
I guess we can be thankful he didn't write a better script, so people at least notice that it doesn't make sense and he has to resort to these explanations. Wanda would be even worse off with a script that made it even halfway convincing that she'd gone evil.