Zechariah was told to prepare the flock for slaughter. The flock is no longer receiving the grace of God but is dedicated to destruction. It has been pointed out by others, and I see it as well, that between Zech. 11:14 and 11:15 is the age of the Church, the time between the two advents of Christ.
In 11:15 Zechariah is told to take new implements for shepherding, “the implements of a foolish shepherd.” The reason is that God had determined to raise up a shepherd over Israel that would not care for the flock at all (v16). This is God’s doing, not the actions of the evil “shepherds of Israel.” This shepherd is “worthless,” as he leaves the flock, being unable because of weakness and blindness to give what the flock needs (v17).
This is the end of the “burden” of the LORD in Zech. 9-11. Ch. 12:1 begins another prophetic “burden,” continuing through Ch. 14. In 12:1-13:6 you will see a message of hope and salvation. God will use Israel to bring the nations to His place of judgment (12:1-9). The people of Israel will see the Messiah they pierced and will repent of sin (12:10-14). Idolatry will be removed from Israel (13:1-6). The story concludes in Ch. 14, but the issue of a shepherd for Israel arises again in 13:7-9.
What we have in the three verses is a short version of what will happen. First, the LORD’s Shepherd must give His life for the sheep. The LORD who said “prepare the flock for slaughter” now says, “Strike the Shepherd,” My Shepherd.
7 “Awake, O sword, against My Shepherd, Against the Man who is My Companion,” Says the LORD of hosts. “Strike the Shepherd, And the sheep will be scattered; Then I will turn My hand against the little ones.
Then the work of the Good Shepherd must be applied to the flock.
8 And it shall come to pass in all the land,” Says the LORD, “That two-thirds in it shall be cut off and die, But one-third shall be left in it:
9 I will bring the one-third through the fire, Will refine them as silver is refined, And test them as gold is tested. They will call on My name, And I will answer them. I will say, ‘This is My people’; And each one will say, ‘The LORD is my God.’ ”
In these verses we see the history of the Shepherd and the flock from the cross of Jesus until the saved nation enters with Him into His righteous Kingdom.
When Jesus told His disciples that all “the Son of Man indeed goes just as it is written of Him,” He knew that this included the evil intentions of the evil shepherds of Israel. The OT was very clear about this.