the right to bear arms?

The right to bear arms argument presented by a lot of my American friends,after one of the deadliest mass shootings in U.S history confuses me and makes a lil angry. I asked how; after a young man was able to acquire body armour and thousands of rounds of ammo,then murder innocent people does the American public still follow a legislation written in the 18th century? I also find it hypocritical to suggest that a free America is one that holds the right to bear arms in high regard, but wants to bomb countries like Iran for bearing arms. The mass murder of people has a very low probability of happening in Botswana because the right to bear arms isn't recognised as a human right or part of our constitution. In a democratic society our rights should not infringe on the rights of others,and certainly NOT endanger the lives of others. The right to bear arms clearly allows those that choose to bear said arms to be a danger to those that choose not to bear them. I think America is a great country, probably the greatest but for them to treat a constitution written by a group of men in the 18th century as gospel today in the 21st century is; with all due respect; a little ridiculous and unintelligent. The U.S criticises nations for not changing their constitutions to be more democratic but they refuse to revisit a constitution that allows certain individuals to buy automatic weapons? Let's be honest, only a militia or terrorist would need that much fire power, and anyone who feels they need that much fire power isn't going to do anything positive with it. Trayvon Martin would be alive if George Zimmerman didn't have the right to bear arms. when will America learn?