Welcome Home Brother Charles (1975)

A proud black man does time in jail where he is subjected to heinous experiments. Once released, he goes about extracting vengeance on those who put him in prison. This includes both seducing their women and murdering them by a mysterious means.

Then the hook is revealed - his murder weapon is his own genitals which are now enormous and super powered as a result of the experiments he endured in prison. You heard me right; he kills with his penis and you see it all!