Adolescent cautionary tale movies are a generational rite of passage. My parents had films like “Last Summer.” I had Tim Hunter’s “River’s Edge.” Millennials got Larry Clark’s dirty old man farce, “Kids,” and Gus van Sant’s “Elephant.” Writer/director Danny Madden’s “Beast Beast” is Gen-Z’s turn in the barrel, and like its subgenre brethren, it’s more likely to traumatize the parents than their kids. If you’ve lived long enough, or have seen enough movies, you can easily predict every single thing that takes place here, including its jarring tonal swing. This is an iffy proposition when you discover one of the three multi-ethnic protagonists is 24-year-old Adam (Will Madden), a White gun aficionado who lives at home with his parents and a lot of weapons. A cynic like me was just waiting for him to shoot somebody, and even if he doesn’t, the film puts the idea in your mind with its soundtrack noises, engendering a sense of unease.