I do think that lethal force should be used to defend a border, but not as a first resort. Firstly, there should be warning signs in multiple languages indicating lethal forces authorized. Secondly, there should be patrols that tell people to go back when found, and lethal force would only be used on people who try to make a run for the border.

And thirdly, because we've had such an open border, there would need to be several years of intense guarding and non-lethal rejection so that the word gets out that the border is closed to undocumented and otherwise illegal immigrants. That way there are no mistakes.

Developed Nations across the world such as Israel use lethal Force to defend their borders from drug cartels, human traffickers, and otherwise ill intentioned people who can't get in legally. There's no reason we can't.