Why have a border? What is the purpose of a border? It is to keep people out because you think they are evil, will drink the milk out the bottle from your fridge, look ugly and smell bad too. Why should poor countries be stuck with keeping the bad people when they would do more damage? At least the richer countries can catch the criminals and jail them or something.
Gotta look out for "your own" .. but is that even moral? Does God value the human life of a person in one country over another? You can allow the deaths of thousands to save a few of your own? How is that moral?
Why would you lock yourself out of interacting with foreigners? That will only breed mutual hatred and an arms race. Keeping people out is not sustainable. Eventually the other countries will want in. And there will be a big war. . Eventually there will be a war, that is guaranteed. At some point you are going to want something from another country and they will refuse to give it to you. Or vice versa. They will want something of yours, you will refuse to give it .. and they will try to take it. Then you will war.
The only sustainable future for humanity is for everyone to get along with each other. This is only possible through interaction, through everyone understanding the concepts of free speech, freedom of religion, right to a fair trial before punishment, right not to be tortured etc.
Its true a lot of third worlders don't understand these concepts, and you are afraid they will bring it to your country. But how does isolating these countries help? How will they learn these ideas if not through interaction? If you are cool with them never learning the values of freedom, eventually they will war. They will be ruled by dictators who build big weapons and will eventually try to take your stuff by force .. it may take 50 years or 100 .. but they will .. and with the nukes of today if even a few get through it's devastating.
Ever noticed that all the places that have ancient border walls are now tourist attractions that you can visit from both sides and still be in the sea country. It's not like the Great Wall of China or Emperor Hadrian's wall demarcates a border to this day .. What we have are failed empires.