That's as bad an example as it can get...
The gunman was identified as 29-year-old Omar Mateen, an American born in New Hyde Park, New York. His parents were Afghan, and he was raised as a Muslim.
You seriously believe that stronger vetting at immigration would have prevented this?
How so?
In this case, from what we know, that guy didn't even have an extremist background, he lived a very typical, if not stereotypical "boring American suburban" "picture-book" life. Before this guy snapped (for reasons we still don't 100% know), they could've picked him as the prime example for "successful integration", DESPITE him being Muslim.
It's indeed a good example for the idiocy of Trump's "proposal".
* Explain to us, how if they back then vetted his parents, whether it would've been likely they forbade them to enter...thus preventing his birth here...thus preventing this attack?
Extremely far-fetched, wishful thinking, entirely in-effective and idiotic - like "the wall".