The person! Regardless of which nature is responsible, the person does the sinful thing.
I don't mind conceding that the flesh is what makes us sin (after all that is my position as well). But we are who we are! (Proverbs 23:7)
Imagine a man who is a Christian. Lets call him Fred. Fred is a Christian and he is in a department store with 4 other customers Bill, John, Peter, and Ted.
Fred grabs an iPod off the shelf and slips it into his pocket and he intentionally leaves the store without paying for it.
Out of the five men in the store, which one sinned?
A. Fred
B. Bill
C. Peter
D. Ted
E. None of the above.