Why does riku turn evil?
Riku might have been Soras best friend yes, but he wanted a great power the knid that could only be obtained by kingdom hearts. Thats why riku turned on sora. To acheave power greater than any other being alive. Sora had fought riku and beat him but then riku became ansem. After sora beat ansem he was sealed in the realm of darkness. But…If you Are talking About KH2 Then Well…He Didnt Turn Bad He Just Turn Into Xhannenhort To Use The Power Of Darkness…