The Assyrians were to be Yahweh's rod against Israel, his own people. They were called upon to inflict punishment for Israel's waywardness and rebellion against God's rule over their lives. Yet, the Assyrians thought themselves masters of their own destiny, powers in their own hands, to invade, conquer and destroy Israel. For that, the Assyrians would also be punished for their arrogant excesses.
God uses people as his instruments unto others, whether his instruments are good or wicked, and whether the objects of his work are good or wicked. As for me, I must be careful to be a useful instrument in his hand, fit for noble purposes (2 Timothy 2:20-21). God might use me to strengthen the discouraged or to rebuke the complacent. Either way, unlike the Assyrians, I must understand what his will is for me in those roles to which I have been appointed so that I do not overstep my delegated authority and do that which is not his will.