“Now at that feast he released unto them one prisoner, whomsoever they desired.”
Mark 15:6 — Greek Interlinear
Church Fathers on Mark 15:6
Pilate furnished many opportunities for releasing Jesus, in the first place contrasting a robber with the Just One.
Which indeed he was accustomed to do, to obtain favour with the people, and above all, on the feast day, when the people of the whole province of the Jews flocked to Jerusalem. And that the wickedness of the Jews might appear the greater, the enormity of the sin of the robber, whom they preferred to Christ, is next described.
No one can feel it a difficulty that Matthew is silent as to their asking some one to be released unto them, which Mark here mentions; for it is a thing of no consequence that one should mention a thing which another leaves out.