I think they're surprise was more of just being shocked that he was capable of stabbing his friend in the back.
This troper has managed to avoid the films, but in the stage productions I've seen, they're typically depicted as the best of friends, and it's not so much "someone's gotta stop Jesus" as it is "Dude, you're in way over your head!"
Two things I've never gotten... First, Mary Magdalene saying to Peter: "It's what he told us you would do/ I wonder how he knew" when she wasn't present for his prediction that Peter would deny him thrice. Second, Judas making numerous references to how badly they have beaten Jesus before they actually hurt him.
I'm pretty sure by the time we see Judas in that scene Jesus is already lying in his cell looking pretty battered up. Though that might only be in the 2000 version
Some productions correct this by featuring her at the last supper (in itself a headscratcher, I'd argue), and the priests' lyrics hint at a conveniently off screen pre-beating beating.
Why does Judas say "If you'd come today you could have reached the whole nation/Israel in 4 B.C. had no mass communication"? B.C. literally means Before Christ, so this story, about Jesus Christ, obviously took place after 4 B.C.
Because the song refers, not to the moment when Christ died, but when he was born and arrived on Earth. The authors have Shown Their Work, because most scholars agree that Jesus Christ was born at some point between 6 B.C. and 4 B.C. (if the Biblical narrative is to make sense, given that, according to our chronology, Herod the Great -he of the massacre of the innocents- died in the year 4 B.C. Matthew and Luke both wrote that Jesus was born during the reign of Herod the Great, which means that, in that case, Christ would have had to be born in the year 4 B.C. or before).
"We have no law/ to put a man to death." You know except for the part where Judaism totally does have the death penalty: Four different types of execution, actually, with the type of execution being determined by the crime committed. Also, one of the crimes for which there is the death penalty is false prophesy, which, haven't you already convicted Jesus of that? Seriously, what's going on here?
They are under Roman occupation and answers to Roman law and court. Not doing so would be... unwise.