So this has a bit of a complex story to it. To summarize the best I can, this is an AU concept where Karma becomes a villain like fully evil on the d&d alignment chart. It happens when her Auntie Betty is killed by King of Galar's soldiers due to her history with crime and the sisterhood. Her passing triggers Karma to evolve since Banettes existence is based around grudges, hatred, agony and all sorts of strong negative emotions.

Karma goes down a path worse than ever before and probably meets her demise by trying to mega evolve to get revenge and being unable to handle all the power. It’s the bad ending to Karma’s life.