Why was the 2nd amendment added to the constitution?

The Second Amendment was added because in order to maintain a free state the people must have the right to keep and bear arms, "armed citizens is what keeps the government honest." The people wanted the Amendment because no government would try to take over with armed citizens.

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. On December 15, 1791, the Bill of Rights (the first ten amendments to the Constitution) was adopted, having been ratified by three-fourths of the states.