The 2nd amendment is intended to protect us from government tyranny. It does not give us the right to carry guns in public places.
The 2nd Amendment is one those confusing amendments because it was written kind of ambiguously.
At the time everyone had firearm of some sort, it was the norm back then. A lot of people are quick to point out that the continental army was small after the war and that they believe that amendment was meant for military people. However the logic in that is kind of flawed because what kind of government says "In this bill of rights we gave ourselves the right to draft an army and give it weapons". Your in charge, your the boss, you don't need permission to go draft and supply an army, you just do that on your own. People also like to say that it's for the right to overthrow a repressive regime because the founding fathers said something among those lines. Other's say it's right for the people to keep and bear arms.
If you ask me, I think people have the right to own and keep arms for self defense and recreational shooting. There are also state defense forces which are sanctioned militias that exist in addition to the national guard.
Couldn't someone legally kill him for carrying a gun around like that?
only if he meets the conditions in the use of force continuum.