Let me see if I can put the Second Amendment in a different light, starting with the exact words:
"A well regulated militia being necessary to the security of a free state, the right to keep and bear arms shall not be infringed."
Let's focus on the first part. It basically asserts that threats against the freedoms we codify and cherish can come from anywhere at any time, be it a foreign power from without or an insipid movement from within. This in mind, it is necessary to have a militia ready to face that threat.
Now note the wording here: militia, not military. Why this specific terminology? Simply put, the threat may come from the military and/or the government it fights for, as was the case during the American Revolution. Say the Second Amendment didn't find its way into the Bill of Rights, and firearms restrictions such as those in Britain were imposed in the US - no harm, no foul. Now suppose Alexander Hamilton was elected president, decided the Bill of Rights was more a set of guidelines than actual rules, and started implementing decidedly imperial policies. Americans don't like this and are ready to take up arms against him...just one problem: they don't have any.
Aside from the fact that Hamilton never ran for president and the scenario is entirely hypothetical, it should be understood that this possibility is exactly why the Second Amendment exists. And as unlikely as that scenario may seem to some people today, it is nonetheless still possible, especially in a political climate where citizens are fearful, ignorant, or apathetic (as in all three for the past decade).
Thus, the Second Amendment exists primarily as a safeguard against totalitarian rule by the government, and against criminals and hostile invaders secondarily. Negative consequences can and do arise as a result, but it is a struggle we take upon ourselves because of the protection that right to arms provides.