@ash Based on this passage in Luke it appears that the chief priests were afraid of the crowds, so they needed Judas to lead them to Jesus when He was alone with His disciples - they needed an insider who could find Jesus when He had retreated to less crowded places. The soldiers themselves may not have seen Jesus - at least up close - remember, no photographs or wanted pictures back then.
Luke 22:2-6 - and the chief priests and the teachers of the law were looking for some way to get rid of Jesus, for they were afraid of the people. 3 Then Satan entered Judas, called Iscariot, one of the Twelve. 4 And Judas went to the chief priests and the officers of the temple guard and discussed with them how he might betray Jesus. 5 They were delighted and agreed to give him money. 6 He consented, and watched for an opportunity to hand Jesus over to them when no crowd was present.
Regarding Judas’ betrayal, here are the two passages that refer to Judas’ betrayal:
Zechariah 11:12-13 - I told them, “If you think it best, give me my pay; but if not, keep it.” So they paid me thirty pieces of silver.
13 And the Lord said to me, “Throw it to the potter”—the handsome price at which they valued me! So I took the thirty pieces of silver and threw them to the potter at the house of the Lord.
Psalms 41:9 - Even my close friend, someone I trusted, one who shared my bread, has turned against me.
The word ‘fulfilled’ in the NT is used in a number of different senses - something could be a literal fulfillment of a prophecy or it could be a type of something in the OT. For example, Jonah was a type of Jesus in that he descended into the belly of the whale 3 days just like Jesus was in the tomb 3 days before resurrection.
In the passage you quoted in Mark, Jesus may have had the entirety of the OT in mind, which points to Christ. Isaiah 52:13-53:12 would be a prime example of a passage that clearly points to Jesus paying for the sins of the people and yet defeating death.
This book goes into far more detail on how the OT is Messianic: