what was I, that I could withstand God?
= = = =
I do construe Peter's "could" in that sentence as involving a degree of choice. He was NOT ABOUT TO withstand God so obviously communicating so forcefully to him.
I don't think "could" in that sentence is 100% and only about capacity.
So you think then that Peter, in other circumstances, would go about purposely defying God? In what things do you suppose?