I want to hit someone with a ball
What would make you want to hit someone with a ball?
Just for training. I want to be a professional at football.
There are many better ways to train for becoming a professional footballer than by throwing balls at people.  Training like you're suggesting may end up hurting the people your hitting with those balls, which is wrong.  Try doing exercises or researching a training regimen!
That all sounds like taking the long route to what I want to do. I'm just going to do it my way.
For what you want to be there are no shortcuts so your plan to do it your way is likely not going to work. But more importantly you're hurting people to get there and that isn't okay. How would you feel being on the receiving end?