There was gang violence in Aang's day too.
Everything's relative. Back then we had Yakone; today we have a proliferation of many gangs. Like I said, the first episode of the series made a point of showing Korra everything that was wrong in Republic City, including gang violence, in order to underline to her that it wasn't living up to Aang's vision.
The membership of the council is essentially random (apart from the Air Nomads, when the head of the family is able to take their seat) in terms of who is or isn't a bender, so the fact that all five were benders during this series isn't indicative of anything systemic (beyond that the governments in the Avatar-verse are autocratic, but that's not something at issue in the series, and there are a bunch of good reasons why that's almost certainly not going to come up).
Says who? Yes, we know from the flashbacks that the Council isn't required
to be all benders, and I'm sure the same is nominally true today, but given everything we've been shown about the intense bender/non-bender tensions in the city, it seems naive to think the all-bender composition of the current council is just a random coincidence rather than a symptom of the pervasive social injustice that's emerged since Aang's time.
Given that the other three councillors were consistently just rubber stamps for Tarrlok, I'd assume it's likely that he manipulated things to put them into office. And he deliberately stirred up the bender/non-bender tensions to gain power, the same way that politicians often stir up ethnic tensions and fears to rally voters behind them. So he probably arranged to stack the Council with benders. Not systemic, no, but definitely a symptom of the social problems in the city, the problems that Tarrlok both exemplified and consciously exacerbated.