Guess I'm similar to, gangrene when I'm, angry then I'm
This is referring to the previous line where he is “bad to the bone” at rap. Gangrene is a disease where tissue dies and could lead to amputation, so he’s metaphorically (for litteral “good at”) bad/rotten to the bone to a point needing amputation.
Gangrene causes the skin to decay and become blackish-yellow/green in color. Sets up next reference, since Bruce Banner only transforms into Hulk when he’s angry and becomes a green behemoth.
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