Am I a bad person for wanting to refuse to be adopted by my stepfather?
It depends, why do you want to refuse being adopted by your stepfather?
I want to be adopted by my mother instead.
It is okay if you don't want to be adopted by your stepfather, even if it hurts his feelings. Adoption is difficult and you should be comfortable with it.
It's a good thing my stepfather's not my real parent then.
You should do what you think will be the best for you and your happiness, that's what matters the most.