Since America is the worst country in the world (according to the lefties), then shouldn't we, as humanitarians, prevent migrants from coming into the country? Won't we be doing them a favour? Since America is the worst place in the world, they're better off in Mexico, or wherever they came from, right?

You can also say that refugees don't exist. In order to be a refugee, you have to be fleeing to a better place. Well since America is the worst place in the world, then there's no such thing as refugees coming to America.