I saw guns in USA

I noticed that people can buy a gun in Walmart, a general store in USA, which is an impressive incident for foreigners whose country forbids to have a gun. The guns are arranged on a shell as like fishing rods. Bullets are put inside a glass show-window that needs clerk's help when people buy them. I checked the weight of a gun, which was not heavier than I had expected. That looked like a toy. Can they kill people easily, actually?