Honestly, simple revenge.
He saw Wakanda, a nation that escaped slavery, the world wars, poverty, starvation, cultural destruction, and calls himself disgusted by their Isolationism.
But that isn't what drives him.
What drives him is his father, lying dead on their apartment floor, panther claws stuck in his chest.
No matter how much he grew, how much he killed, or how big he tried to make his goals, it all boils down to a boy cradling the corpse of his father.
He wants revenge, revenge on a world that would do this to him, and if he has to dress it up with some vague goal of taking down the white man, if he has to fool himself into thinking that his actions will be the salvation of his people then so be it.
So long as the world hurts as much as he does.