These things don't just come, arrive and settle like a bird picking up a few bits of crumbs. They develop. I think the best word for these things is develop. They develop because of the human beings who just happen to be there at the time.
A surging, seething, murmuring crowd of beings that are human only in name, for to the eye and ear they seem naught but savage creatures, animated by vile passions and by the lust of vengeance and of hate.