exactly why her going after midori makes 0 sense, she should have teamed up and kill the org
I agree, but Midori decided to keep escalating instead of trying to talk Nezumi out. Midori's self destructive tendencies at her worst.
I guess Nezumi didn't feel confident enough to turncoat right after returning from the ship, perhaps fearing for Ao's safety (he had basically just returned from the dead).
And she was right, they used him to force her to kill Midori. Now Ao is here at the beach with her, safe and sound, so nothing holds her down anymore. And now it's personal