The second amendment needs to exist because dispute the unlikelihood there is always the chance the people might need to, and will even be required to by the constitution, take it upon themselves to overthrow the government. To do so the people must have the power, in this case the weapons, to overcome the old government (which has a military) and this is the reason we have a second amendment. How we choose to control weapons is really another matter and up for debate on a number of fronts, but the necessity of the core amendment is obvious.