The only justification for what was done with Wanda is that it will play into a bigger issue down the line which we have yet to seen. I think they could have had her be the villian but switched up how it was done. I think a more palatable twist is if we found out that a Scarlet Witch from another 838 was influencing her. 838 SW had become completely corrupted by the Darkhold but was defeated by the heroes of that universe and imprisoned in some pocket dimension where her magic was useless. When our Wanda started using the Darkhold at the end of WV, she was able to connect to 838 SW whom then found a way to influence her. From there her mission would be to find the source of ultimate power and she needed America Chavez so she could gain the power to cross dimensions and free 838 SW. At that moment 838 SW would turn on her and try to steal her power to buff herself and Wanda and the other heroes would join forces to take her out. Alot of the same beats that happened in DS2 could still occur, just we wouldnt have the sloppily handled children angle.