A.T. Robertson Commentary Mark 14:10

"And Judas Iscariot, he that was one of the twelve, went away unto the chief priests, that he might deliver him unto them." — Mark 14:10 (ASV)

He that was one of the twelve (ο εις των δωδεκα). Note the article here, "the one of the twelve," Matthew has only εις, "one." Some have held that Mark here calls Judas the primate among the twelve. Rather he means to call attention to the idea that he was the one of the twelve who did this deed.