How much of that is really on Taylor though? He had to work his story within the boundaries of the video game, where Diana was kinda just evil. Which given that Lex is a good guy, works just fine. This is also why I give him slack with the Harley Quinn thing. It was strongly implied in the game that Harley helped Joker destroy Metropolis and Batman works with her anyway.
I think Taylor did the best he could to add more substance than the premise had any right to have.
Anyway I’m never offended when my favorites have evil alt doppelgängers, for a franchise like dc where part it’s appeal is exploring what ifs? Stories like this come with the territory.