Assyria is the tool of judgment upon Israel, chosen by God to accomplish His purposes - in spite of her arrogance, idolatry, blasphemy and wickedness.
But the Lord did choose Assyria (10:5-6) to "tread (Israel) down like the mire of the streets." This, however, did not absolve Assyria of her own sins, and that which is charged as the greatest sin, is that of arrogance (10:15), "shall the ax boast itself against him who chops with it?"
There is so much to see here regarding the sovereignty of God and the responsibility of man, how they do not contradict one another but are held together gladly in the wisdom of God.
For us as well, it is critical that we note the great sin of arrogance and self-boasting pride. Paul warns, "But God forbid that I should boast except in the cross of our Lord Jesus Christ, by whom the world has been crucified to me, and I to the world" (Gal 6:14).