Every time I hear somebody talk about how much murder, gang violence, etc. is going on in some other country (what Trump called a "shithole" country) and so we should absolutely not allow immigration from that country to America, I think about that evil scientist or bureaucrat in every zombie movie who gleefully shuts the door behind them, leaving some hapless guest alone in a fortified room with a zombie, who then tears them to shreds while the scientist/bureaucrat watches through a window.
If we don't let those people come here so that they don't get killed over there, we ARE that evil/scientist bureaucrat. The only difference between the hapless guest getting torn to shreds and us is that we had the good fortune to find ourselves on this side of the door.
And yeah, you could respond to this metaphor by saying "But we can't open the door! The zombie will follow them through!" To which I must remind you-- there are zombies on this side, too. The scientist/bureaucrat might even be one of them-- he often is in the movies.