This is a clever little film. It looks like a frothy, fast paced 'teen movie', which it is. On the other hand, without being at all obvious (until the very last sequence) or preachy, it also raises serious questions about escalating addiction and taking things too far and mass complicity and responsibility and the consequences of watching and doing nothing.
We may think "It's a movie. It has nothing to do with my life. I would never get myself involved in something like that, I would intervene".
But hang on, most of us, at one time or another have been, are and will be, watchers who fail to stop something that shouldn't be happening. If it is only (for instance) failure to intervene in a case of racial abuse on a bus.