Second Amendment, ratified in 1791, was proposed by James Madison to allow the creation of civilian forces that can counteract a tyrannical federal government.
Abundant historical evidence indicates that the Second Amendment was meant to leave citizens with the ability to defend themselves against unlawful violence. Such threats might come from usurpers of governmental power, but they might also come from criminals whom the government is unwilling or unable to control.
Maybe try reading the Constitution and it's amendments slowly and it may help your comprehension of the document.