Yes, the people who create the programs that allow users to copyright other people, are the people that need to be blamed. If you make a network where copyright is available, the people are going to copyright others works, and the creators know this all along when they make it.
Yes, P2P network creators should be held responsible for copyright violations of their users, because the P2P network creators know exactly what they are doing when they set up their networks. It is hard to even think of what legal things are shared on those networks. Those who make those networks know they are helping people violate the law.
No, not at all. P2P network creators should not be held responsible for the copywrite violations of the users. They have simply provided tools in which to gather information and files. There are files that can be acquired legally through torrents and file sharing. It is up to the user's discretion whether or not it is used in a legal manner.
If creators were held liable for what people did with their inventions, nobody would ever create anything. Gun manufacturers would never build a gun if they were responsible for someone getting shot. Car companies would never build a car. Could you imagine their liabilities? Medicine? Sporting goods? Everything can be used incorrectly to harm others. I creator is not responsible for what a user does.
Even though they created the environment for copyright violations, that doesn't mean they should be held responsible for it. By that logic, gun makers should be held responsible for criminals that use their guns to commit crimes. Why not charge the ISPs that provided them the Internet connection? Why not arrest the manufacturer of the computer they used? It's not any different. Only the user should be held accountable.