A.T. Robertson Commentary Mark 15:12

"And Pilate again answered and said unto them, What then shall I do unto him whom ye call the King of the Jews?" — Mark 15:12 (ASV)

Whom ye call the King of the Jews (ον λεγετε τον βασιλεα των Ιουδαιων). Pilate rubs it in on the Jews (cf. verse 9). The "then" (ουν) means since you have chosen Barabbas instead of Jesus.