What motive did the founding fathers have for adding the right to bear arms into the constitution?

The right to bear arms is not the whole thing. In fact if you read it it is quite self explanatory. The second amendment states "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." So the right to bear arms is so we can have a well organized militia.