OME say it was a bit more than a century after Jonah ministered in Nineveh that God issued a declaration of judgment against that great city. He did it through the mouth of the prophet Nahum. God had threatened violence against them before, but their repentance and humility had been enough to elicit His grace upon them. That is the story in the book of Jonah. The generation of Assyrians who had heard Jonah preach before had all passed away though, and the Assyrians, as a people, had altogether forgotten God and His grace. As such, Nahum was sent to prophesy against them. God's nature was as good as ever, but He most definitely reacted angrily against the Ninevites.
We know that the Assyrians were used by God to punish Israel, but they sinned even in their participation in God's plan (not that they understood God's plan, that is, how He had chosen to use them against Israel). In the first place, the brutality of the Assyrians was extreme. Now, it might be argued that since hell is rather brutal, one might reasonably ask if brutality is intrinsically evil? Here is difference though. The Assyrians were brutal without a just cause. In fact, they rather relished their opportunities to be violent. Even when God's holy justice drives Him to punish His creations with severity, He never enjoys inflicting pain. The Assyrians were warped to the point of enjoying the pain of people who to them were innocent. This was grossly ungodly.