Aglin is the former leader of The Hoods. His wife died in the Glades Earthquake, and he soon formed his gang drawing inspiration from the Hood. He, along with the rest of the Hoods, were taken down by the Arrow and placed in prison.
After the Undertaking, Aglin and the other members of The Hoods, copied Oliver's tactic into scaring one percenters to stop taking advantage of people but they were much more violent. They eventually killed the current mayor, which was a ploy for Sebastian Blood to run for the now vacant mayor spot.
It is unknown what he was like before the Undertaking, but after the death of his wife, Aglin became angry, vengeful and unstable as he was ruthless in killing people he saw as responsible for the quake, such as Oliver Queen who had nothing to do with the Undertaking, and he tries to justify his ruthlessness as acts of justice.
These negative traits of his are what eventually led to Aglin's deserved/justified incarceration.