Albert Barnes' Notes on the Whole Bible - John 21:21
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.... read more
The Pulpit Commentary - John 21:21-22
Curiosity rebuked. Peter and John were the two among the twelve who were nearest to Christ, and they were peculiarly intimate in their friendship and congenial in their disposition. It was very natural that, when the risen Jesus had uttered so explicit a prediction concerning the future of the apostle—viz., that he should live to old age, and then should glorify God by enduring a martyr's death by crucifixion—a general desire should be aroused in the breasts of the disciples to know... read more