"Mark" (editor) means to prove that that distinction between Jesus and Christ has not to be made. John the Baptist was not the true Christ, he was only the precursor of the authentic Christ.
It is the latter who was crucified, not John.
Hence, John was Bar-Abbas, the rival Jesus about which some people claimed that he was the true crucified "Jesus son of Father".
- Valentinus preached that Paul had known Theudas.
- Paul could have known only the Risen Christ.
- Hence, for the valentinian Paul. the risen Christ was Theudas (under the mystical name of 'Jesus').
"Mark" (editor) invented the decapitation of John the Baptist because he derived it from the fate of Theudas, considered the same Risen Christ (the one known by Paul, per Valentinus).