A lot of the rage was from their entire race being literally enslaved by the Empire, so they were angry, as anyone would be. Also, a lot of the "intimidation" scenes from the OG movies were when Chewie was trying to appear thus.

But also like their whole thing is to live in harmony with nature and have honor, so e.g. they all have retractable claws (not joking), but you never see those b/c they refuse to use them against a fellow sentient being, even when immensely angry. Edit: if it helps to know, they are for climbing.

So if anything I think their culture, at least pre-Empire enslavement, would bias them towards the Light side. Though literally everyone, and especially in the face of such evil being directed at them, would face great temptation to use all that power granted to them to achieve an end that they considered "good" - e.g. wiping out the "evil" enemies that enslaved them, as in every... last... one...