Light Yagami didn't kill Teru Mikami. I suggest that you rewatch the final episode or final volume of the manga. To save you the trouble, Mikami stabbed himself with the pen he had to give Light a chance to escape. In no way, shape or form did Light kill Mikami. It was purely Mikami committing suicide to help his 'God'. Also, He dies when he's imprisoned in the manga. Harakiri. YAY!