The Second Amendment to the Constitution in the Bill of Rights. It says, at the end, ". . . shall not be infringed." Period. Do we think for a minute that there weren't felons, mentally ill, and others we now infringe upon?
It doesn't say the "shall not be infringed after you pass the never ending restrictions that Congress may pass at their whims." or some such. I mean, we actually have to ask the .gov for permission to buy a firearm in this country. They would never have stood for it back then.
Granted, many murderers, rapists, robbers, etc.; you know, real felons, were usually executed back then too.