I wonder if the city of Rome during Republican and Imperial times had some kind of "bad district", the typical area that you would avoid during the day and definitively never go in the night, just like it happens in modern big cities.
In case that yes, which district it was? Where was it? Why was it that bad?
I also wonder if they were language-based or ethnic-based guettos like it happens in modern times, as I suppose ancient Rome had immigrants.
Rome didn't really have ethnic districts, though Alexandria to an extent seems like it did. We do know about less...classy districts in the city however. The Subura, in the little valley between the Viminal and Esquiline, is notorious in our texts as a den of prostitutes, beggars, and lowlives in general. But I would hesitate to call it the hood. Much is often made of the Subura these days, but despite attestations as to its generally seedy character (particularly by Martial) the Subura was also a place of bustling trade and we're told of the skilled workers who set up shop in the Subura and the delicacies that were sold there. More than a few wealthy, powerful people had houses in their--Caesar, famously, until his election to pontifex maximus granted him a pontifical house elsewhere, but also for example Lucius Arruntius Stella, consul of 101. No doubt there were other neighborhoods with similar reputations, particularly among the vici--some vici must have been considered safer or better than others--but we simply don't know about them