Albert Barnes Commentary Zechariah 12:9

"And it shall come to pass in that day, that I will seek to destroy all the nations that come against Jerusalem." — Zechariah 12:9 (ASV)

In that day, I will seek to destroy - Woe indeed to those whom Almighty God shall “seek to destroy!” Man may seek earnestly to do what he ultimately cannot do.

Still, it is an earnest seeking. And whether it is used of human seeking which fails, or which succeeds (1 Samuel 14:4; 1 Samuel 23:10; Ecclesiastes 12:10), initial or permitted (1 Kings 11:22; Zechariah 6:7), it is always used of seeking to do what it is a person’s set purpose to do if he can.

Here it is spoken of Almighty God. Ribera states: “He says not, I will destroy, but I will seek to destroy, that is, it shall always be My care to destroy all the enemies of the Church, that they may in no way prevail against it: this I will do always to the end of the world.”