" Later, because of their warlike raids, the name came to mean brigands or bandits"
Peter, I think you may be falling into the trap of the Roman propagandists.
I think that there is scant evidence that the Brigantes were any more or less warlike than any other British tribe before or during the Roman occupation.
What I find interesting is the person who led the Brigantes, Cartimanua, was a woman. It would be interesting to speculate on the role of women during the Iron Age and how this could possibly have related to earlier times.