John 21:21 - Albert Barnes' Notes on the Bible

What shall this man do? - This question probably means, “What death shall he die?” But it is impossible to ascertain certainly why Peter asked this question. John was a favorite disciple, and perhaps Peter suspected that he would have a happier lot, and not be put to death in this manner. Peter was grieved at the question of Jesus; he was probably deeply affected with the account of his own approaching sufferings; and, with perhaps a mixture of grief and envy, he asked what would be his lot. But it is possible, also, that it was from kindness to John - a deep solicitude about him, and a wish that he might not die in the same manner as one who had denied his Lord. Whatever the motive was, it was a curiosity which the Lord Jesus did not choose to gratify.