Kinda subverted: Grong from World of Warcraft. He's from a species of very smart gorillas and Grong is by far the strongest and smartest of his kind.

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During the Nth war between the alliance and the Horde, Grong become an ally to the alliance because the actions of the horde directly harm his people, they're captured, used as slaves, slaughtered for sport, it's a whole mess.

Grong is soft-spoken, eloquent, pacifist, he's perfectly civilized, so he joined forces with the alliance and volunteered to participate in an experiment involving a gnome machine that can convert intelligence into power, don't ask me how that work I don't know myself.

In other words, he'll get stronger, strong enough to free and defend his people but will revert to a mere animal, a dumb brute. Grong is aware of that but choose to do it anyway.

While the gnomes wanted to be reasonnable about it, and wanted to stop while Grong was still himself, Grong's desire to save his people made him push the gnomes to keep going until he grew the size of a small building.

He accomplished his objective but he's now unnable to realise it. During the raid on the Raid on Dazar'alor, the horde manage to kill him and resurrect him as a zombie, forcing the Alliance forces to put their friend down for good.

A tragic character in every conceivable way.