Questions

Why did they write in a right to bear arms in the Constitution?

My answer to the question is simply this: WE THE PEOPLE are the sovereign authority of the United States of America.

There are several natural consequences of the political primacy of the citizen. I'll just cover the first two here:

Amendment I

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.

Amendment II

A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.

First, the citizen as the sovereign authority must be free to examine the facts for himself and communicate his conclusions to his fellow citizens. He cannot be compelled to believe a falsehood and therefore act on it.

Second, the citizen as the sovereign authority must possess the physical instrument of political authority and be prepared to use it. The ballot that is cast by the citizen, like the paper currency in common use, must be backed by a physical value.  The government must be subordinate to the sovereign authority, the citizens of the nation. And citizens must have the knowledge and the means to enforce their authority.

Contrary to what the practicing Kantians want us to believe, the first four words of the Second Amendment does not constitute a license to disarm the citizens and render them helpless in the face of a supreme state. It means that the citizens must be armed, trained, and organized to enforce their authority upon the government.  Now no rational person wants a civil war in our nation, but if the government would simply comply with the Constitution it wouldn't be necessary.