Well...remember when I asked you if the Chaos Emeralds blew up in your face?

Apparently that happened to Sonic in my world, and the power drove him insane, as in kill everything he sees insane.

It didn't last very long but it wasn't a one off. That's why Amy had to stop him, but Supersonic had absorbed so much Chaos energy that time that when Amy and the others tried to drain the energy to change sonic back, Supersonic became a separate being.