John B's only role was to portray Elijah, so he has to die before the Transfiguration. That is the initial idea, and all three of them execute it.
The second thread that starts growing is the "Jesus is Elijah" thingy that might have start being sung after Mark, not surprisingly so, given the fact that Elijah and Jesus both revive dead children, among others.
And the third "thread" is that the damned John Presbyter ruins it all by not participating in the beheading scheme, and even cuts short the second thread by having John B emphatically deny that he is Elijah!
Yet bear in mind that John needs to die so the Elijah in him can fulfil his utterly silent and non-descript role, next to Moses, who surprsingly reads from the exact same script, oh and jesus of course, in his "whiter-than-any-bleach-on-this-earth-could-have-achieved" dress. Mark sure was a woman!
Here's the TL;DR version:
https://www.academia.edu/40695711/Absol ... ory_manner
Page 19-49 for the impatient ones among you
Discussion about the New Testament, apocrypha, gnostics, church fathers, Christian origins, historical Jesus or otherwise, etc.