explain to me how the BEEP is this movie a horror?

Because they used the soundtrack to build tension and then do a quick cut paired with a loud noise to scare you: instant horror!

I wouldn't say that it is. It's a thriller attempting social commentary. Not very well, but that's what I'd say it is.

They wear masks. Invade your house. Kill tons of people. Harsh social commentary.

Horror is a very hard genre to put down, because it's so diverse.

You get slasher movies, paranormal movies, found footage movies, horror thrillers, horror comedies, sci-fi horror and so on and so on. It's quite a diverse genre. Gremlins, for example, sometimes gets called a horror movie. But has very little to do with The Blair Witch Project as a movie.

Sometimes things get pegged as horror that probably aren't. I can see why you could argue that The Purge isn't horror, though the premise of legalised murder is quite horrific. You could also argue that the original Saw film is more of a thriller too. The sequels started getting more with 'torture porn' that they became famous for.

It just depends how you classify stuff.