I think their surprise was more of just being shocked that he was capable of stabbing his friend in the back.
Depending upon the production, they're often 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, and the priests' lyrics hint at a conveniently off screen pre-beating beating.
To be fair, the Bible doesn't say she WASN'T present, either. Some sources even say she was an apostle toonote (Leonardo da Vinci may have famously depicted her in his namesake painting).
Occam's Razor: at some point in between the Last Supper and the denial, Peter came to Mary off-screen and told her a variation of 'can you believe what Christ told me? He said I'll deny him three times!'
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.
When roman authority was lacking for a short period there was a whole swathe of executions,
Early in Jewish legal doctrine, scholars took to maintaining such stringent demands on the evidence required to actually enforce capital punishment that people were rarely, if ever, executed. In the first century CE, it was generally agreed that "a Sanhedrin [the Kangaroo Court Caiaphas presides over in the musical] that puts a man to death once in every seven years is called destructive", with some scholars arguing that the limit was more like seventy years.
Why do the priest insist that Judas take the "blood money" when he was willing to betray Jesus for free?
It's not unheard of, both in fiction and Real Life, that a spy/traitor/whatever gets paid even against their will, either as a mean to share the guilt or to partly wash the other person's hands or simply not to owe them anything. There's at least a small amount of leverage if some time later that person comes back asking for a reward and you reply 'we've already paid you.'
The payment also matched the lowest prescribed price for a human being.
For the most part, the musical does a pretty good job at keeping the divinity of Jesus ambiguous. All of his famous miracles happen before the narrative starts (except the resurrection and the ascension, which happen after), leaving it up to the viewer to decide if they really happened or if they are just tricks and rumors. However, there is one thing for which I can't think of a non-supernatural explanation. How did Jesus know that Peter would deny him *exactly* three times?
To be fair, that can also have a non-miraculous explanation, although YMMV on how far-fetched that would be. Jesus knew Peter well enough to predict the way he'd react in an interrogation, so maybe he just, correctly, assumed Peter would be asked about it and he'd deny having any connection to Jesus. The whole 'I knew you would/wouldn't do it' trope (is it a trope?) has been done to death, and arguably it happens a lot in real life as well (e.g., a mother applies reverse psychology to get her kids to do the dishes, as she knows how they'll respond to it).