A.T. Robertson Commentary John 21:19

"Now this he spake, signifying by what manner of death he should glorify God. And when he had spoken this, he saith unto him, Follow me." — John 21:19 (ASV)

By what manner of death (ποιω θανατω). Undoubtedly John, who is writing long after Peter's death, seems to mean that Peter was to die (and did die) a martyr's death. "Whither thou wouldest not." There is a tradition that Peter met death by crucifixion and asked to be crucified head downwards, but that is not made plain here.