There is no need to speak as God so ordained the events in His sovereignty to have a crowd that would choose death. Do you think if God needed people in that crowd to sway the decision He would not have told the disciples to speak up? Silence is the best option because God's will already dictated that moment in time. Some prophecy is a foretelling of the future but other prophecies are ones of which the Lord makes happen (divine intervention). This may be such a case, that the right people were present of which the Lord knew their heart that they would crucify Christ.
The question itself is a rather selfish question, one of self-preservation. The problem is, no one took Jesus' life. He gave it. He sacrificed Himself, willingly. The question questions God's sovereignty, in actuality. It also, in some way, misses the point. To speak up, even knowing the Gospel, and call for His crucifixion one might just feel the burden Judas felt. He committed suicide because his conscience could not bare it. Innocent blood on his hands.
One might think in knowing the Gospel how easy it would be to call for Jesus' crucifixion, but don't forget you know of His great love and character. You know of all that He stands for. How easy can it be to kill love itself? Even knowing Love has decided to sacrifice itself? So pure, so holy. Is it truly such an easy decision? Jesus asked the Lord if there was another way, for the cup to pass. Could you stare Him in the eyes and see His anguish and so easily call for His death and suffering?
Silence is the better option because it acknowledges God's sovereignty.