"This disgusting piece of sh!t didn't use a gun to defend himself against a threat."
Obviously not, but the best chance anyone would have had to prevent him from murdering his girlfriend would have been someone else with a gun.
"The Constitution was never intended to inform the future laws forever and ever."
The Constitution actually was intended to remain the foundational document of the country so long as the country exists. For any necessary changes, a method was created by which amendments could be added. As far as I can tell, there has been no change to the Second Amendment, and therefore, it remains law.