The second clause does not follow from the first. Murder in the US is heavily demographically biased, and most gun owners are not murderers. Furthermore, will you be the one going door to door to collect everyone's firearms? It sounds like you are saying, "Surely, we must do something. This is something; therefore, we must do this."
Using the argument of "Hardened criminals will still get them anyway" is like saying we shouldn't have a law against murder because some people will do it anyway, so it's a waste of time. The point is to deter or make it extremely difficult for as many people as possible.
No, no it's not. You are conflating possession of a tool with misuse of the tool to commit a crime. The US has been trying to make it "extremely difficult for as many people as possible" to do a lot of things, and failed spectacularly. Now imagine that same level of failure applied to something protected in the US constitution.