That was pretty good.
One question: where you getting the plot from Sonic X because if you are, then you did the wrong story of Sonic X. Here's how it goes: there's this giant seed that is taking away the life forces from Sonic and his friends. Its too powerfull, but there was only one choice to destroy it: the Sonic Beam Cannon. They had no other choice but to sacrifice one of the team. Cosmo decided to make the sacrifice. Okay let's get to the point: no one killed her. Eggman didn't kill Cosmo or even Shadow. Cosmo just comitted a sacrifice. That's all. (if you're making a different plot, then ignore what I said okay?)