The legal concept existed for at least a century before the Constitution, as declared in the English Bill of Rights. It certainly was not a new concept at the time of the US Bill of Rights being written.

Think of it this way.

Is it ethical to force someone to be a catholic, or a jew, or an atheist, or a hindu? I think you'll agree it is unethical. Suppose we didn't have the first amendment. Would that suddenly change the morality of forcing someone to practice or not practice a religion? I think we'll both agree it is still not ethical.

So then, the first amendment didn't create a right to freedom of religion. It described a right that already existed. It tries to bring legality in line with morality.

The same is true for the second amendment. Gun ownership isn't ethical because of the second amendment. The second amendment exists because gun ownership is ethical.

If you believe self-defense is a natural right of all individuals, then the right to keep and bear arms exists with or without the 2nd Amendment codifying it into law.