They behave like a pack of unclean dogs. During the day they will often get chased away. But at night they roam through the city, looking for leftover bits of food. Their behaviour fits with the darkness: injustice, unbelief, bitterness. A definite threat for the author.
We encounter the image of dogs as a sign of uncleanness in more places in Scripture. Moreover we encounter dogs as punishment for wicked behaviour, for example, in 1 Kings 14:11 where anyone belonging to Jeroboam are said to be eaten by dogs; or in 1 Kings 21:19 where we read about Ahab who has stolen Naboth’s vineyard by murdering him. He is then told: In the place where dogs licked up the blood of Naboth shall dogs lick your own blood.
And in 1 Kings 21:23 The dogs shall eat Jezebel within the walls of Jezreel.
That is even more than just licking the blood.
The language of the opponents also says a lot: full of poison, like swords. These are images of a deadly threat. Their language is also prideful. This often goes together: proud over against God (Who can hear what we are saying?
) and acting with bloodthirst against fellow men.
6 Each evening they come back, howling like dogs and prowling about the city.