Justice is a relative concept because morality is also relative. There are approximations simply because humankind shares some common beliefs but that is all there is, to think there is such a thing as absolute justice is the same as thinking there is such thing as absolute morality, there is no such thing because they are human cultural concepts.
The point he was trying to make is that bandits are not necessary evil, that banditry might be something they have to resort to survive with that being the point he was going to make before realizing that what they done was very difficult to justify.