I think ryu became evil due to the power of the satsui no hadou that oni has, oni passed it on to ryu making ryu evil, i might be wrong but thats what i think happened, hope i helped
One must only believe that one is immortal, and not designed for any particular purpose, to be evil, like my agnostic Grandfather, who has written so many warped books and manuscripts that he now...
Well Shadow the Hedgehog was first a happy character until people came in and killed his friend Maria. Then he was sent to earth and pretty much hated humanity.