As I understand it, all these criminal gangs started as the paramilitary wings of various political parties, each of whom had their own street thugs. Then the gangs somehow became autonomous of the parties that created them, and then further started to form coalitions among themselves, taking power directly.
Maybe the problems go all the way back to when it was a French colony full of slaves (that successfully revolted, with a degree of violent retribution visited upon the slave owners that would make Hamas look conciliatory), but US interventions haven't helped things (including a certain Hilary Clinton's recent meddling)