Why can people have a gun?

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If you read my discussion before about "a baby was shot in the head" then you will see that the baby is still alive because it's just an air gun not a firearm. But there are many cases in my country that involving misused firearm and the really bad thing is that the one who misused it are someone who have the authority for carrying a firearm, soldier and police. Even they sometimes can't control their emotion. Well how about ordinary people? How can they control thei emotion? I know that americans have the rights for carrying a gun based on second amandement (is it right?), but do you realize that it is really simple to kill when using gun.