There are many examples in the Old Testament of people and nations being instruments of God’s justice only to find themselves later rebuked or destroyed themselves.
Jehu – After destroying Ahab’s family (2 Kings 9,10) which seemed at the time to be accomplishing the purposes of God. We are later told that punishment will come to Jehu’s house because of this (Hos 1:4).
Assyria – At one time instruments of wrath for punishing sinful Israel (Amos 2,3,9; Isa 10) are later to suffer judgement (Nahum, whole book).
Babylon – Again, used to punish Israel but will also suffer punishment.
Were they ever doing God’s will?
Or did God just stop protecting his people?
Or did they go too far?