What do you call it when a group of neglected people spontaneously forms a cruel society?

If you exclude the "traumatic event" part, then "anarchy" works.

from dictionary.com,

4 : confusion; chaos; disorder: Intellectual and moral anarchy followed his loss of faith.

I'd call it a "Lord of the Flies situation". Lord of the Flies is a novel about some school kids on a field trip that are shipwrecked and wash up on an island with no adults, and form just what you described: a spontaneous, cruel society of neglected people.

I think nearly all somewhat-educated adult English speakers would understand exactly what you're talking about; we all had to read it in high school.