Another reasons to consider Mark 1:9-14 as late interpolation.
7 And this was his message: “After me comes the one more powerful than I, the straps of whose sandals I am not worthy to stoop down and untie. 8 I baptize you with water, but he will baptize you with the Holy Spirit.”
9 At that time Jesus came from Nazareth in Galilee and was baptized by John in the Jordan. 10 Just as Jesus was coming up out of the water, he saw heaven being torn open and the Spirit descending on him like a dove. 11 And a voice came from heaven: “You are my Son, whom I love; with you I am well pleased.”
12 At once the Spirit sent him out into the wilderness, 13 and he was in the wilderness forty days, being tempted[g] by Satan. He was with the wild animals, and angels attended him.
14 After John was put in prison, Jesus went into Galilee, proclaiming the good news of God. 15 “The time has come,” he said. “The kingdom of God has come near. Repent and believe the good news!”
9 At that time Jesus came from Nazareth in Galilee and was baptized by John in the Jordan. 10 Just as Jesus was coming up out of the water, he saw heaven being torn open and the Spirit descending on him like a dove. 11 And a voice came from heaven: “You are my Son, whom I love; with you I am well pleased.”
12 At once the Spirit sent him out into the wilderness, 13 and he was in the wilderness forty days, being tempted[g] by Satan. He was with the wild animals, and angels attended him.
14 After John was put in prison, Jesus went into Galilee, proclaiming the good news of God. 15 “The time has come,” he said. “The kingdom of God has come near. Repent and believe the good news!”
(Mark 1:7-14)
If Jesus appears immediately after the arrest of John, then we may read in the scene of the arrest of John an implicit accusation against John: he was a brigand and he deserved to be arrested.
Jesus could well say:
All who have come before me are thieves and robbers, but the sheep have not listened to them.
(John 10.8)
In the same way, even if John came before Jesus and he was a robber (hence the true implicit reason of the his arrest), the Jews have not listened to him, just as to old prophets.
This may explain why the story of the arrest of John by Herod & wife etc was designed to exorcise the accusation against John as robber. He was put in prison unjustly.
But then was John introduced in the narrative in virtue of the his past of Zealot and/or ''Robber'' ?