The longstanding view of John bar Zebedee's life was that he authored at least five texts (the Fourth Gospel, the three Johannine letters, and the Revelation), mentored the likes of Papias, Ignatius, and Polycarp, opposed heresiarch Cerinthus, was miraculously protected from harm when dipped in oil by the Romans before being (temporarily?) exiled to Patmos, before finally dying peacefully of old age in the 90s or early 100s.
This led to my puzzlement when I read the following dialogue in Mark 10.35-40:
James and John, the sons of Zebedee, came forward to him and said to him, ‘Teacher, we want you to do for us whatever we ask of you.’
And he said to them, ‘What is it you want me to do for you?’
And they said to him, ‘Grant us to sit, one at your right hand and one at your left, in your glory.’
But Jesus said to them, ‘You do not know what you are asking. Are you able to drink the cup that I drink, or be baptized with the baptism that I am baptized with?’
They replied, ‘We are able.’
Then Jesus said to them, ‘The cup that I drink you will drink; and with the baptism with which I am baptized, you will be baptized; but to sit at my right hand or at my left is not mine to grant, but it is for those for whom it has been prepared.’
Jesus tells James and John they will drink the same 'cup' that Jesus will drink.
The 'cup' Jesus drinks is mentioned again in Mark 14.36, when Jesus prays in Gethsemane:
He said, ‘Abba, Father, for you all things are possible; remove this cup from me; yet, not what I want, but what you want.’
By any appearance, the 'cup' Jesus drinks is his death as a martyr.
I've read Mark 10.35-40 many times, but one day the full implication suddenly clicked: If Mark 14 really does present the 'cup' as death by martyrdom, then it follows that Jesus was predicting James and John would also be killed as martyrs.
Acts 12 briefly describes the death of James by the year of AD 44 (Herod Agrippa, who ordered James' death, dies later in the chapter). John is not mentioned in Acts after his brother's death, though Paul in Galatians 2.1-10 seems to indicate John was present at the 'council of Jerusalem', circa AD 50, which is depicted in Acts 15.
Despite this, there seems to be a completely separate tradition that dominated Christian literature from the second century onward, that John bar Zebedee never was killed as a martyr.
Outside of Mark 10.35-40 (and Matthew's parallel), are there any ancient references to John suffering death by persecutors, not just a divinely nullified attempt at killing him, followed by exile? Did any ancient writers seek to harmonize the apparent contradiction?