The Assyrians were a formidable enemy, but they made mistakes. They accepted tribute from Israel, then invaded anyway, and they challenged God directly while they were at it.
This made them betrayers as well as destroyers of Judah when a godly king was in power. Isaiah made the point that God took a dim view of their deceit and arrogance; they would reap what they had sown:
Isaiah 33:1 What sorrow awaits you Assyrians,
who have destroyed others
but have never been destroyed yourselves.
You betray others,
but you have never been betrayed.
When you are done destroying,
you will be destroyed.
When you are done betraying,
you will be betrayed.
That Bible blog 