Should I be mean to my step-brother who filmed me in the shower?
Why would he do that to you?
He is just a psychopath, he was so awful to me.
Him filming you in the shower is completely unacceptable. At the same time, it's understandable that you might push back on him for engaging in something cruel like this in a way that sets a boundary on acceptable behaviour.
I was trying to stop him from doing it and he just kept recording it on his phone.
I don't blame you for trying to stop him. I would freak out myself, but rather than take things into your own hands, I think you just tell your parents and have them deal with him. I am sure your parents would make him understand how this behaviour is wrong, and he will understand the ramifications if he is still thinking about filming you.