Really? Because I figured the reason he did it was to show that he could do it. It's like waving a flag that says "Hey, I'm a Devil Fruit user and I can do things that'll give you nightmares for the rest of your life. Don't fuck with me."

Also, I'm pretty sure the cannonball wouldn't have taken his head off first. I...I don't really know what he did, entirely, but I think he had to take the guy's head off to switch it with the cannonball...? I don't know, it'll probably be explained better in later chapters.