The ending is so badly rushed
I wouldn't exactly label The Belko Experiment as a serious film. It's basically a gore fest with an interesting concept...sort of like The Purge films. Except, I enjoyed this film way more than any of The Purge flicks. I do give credit to the plot. It had some potential, but was limited by it's unimaginative writing and bad casting choices (or just bad characters). But like I said, the concept was interesting. For entertainment purposes, I'd say this film is watchable. But don't expect too much depth in the story-line. What bothered me the most about the film, was the badly rushed ending.
After Mike survives the experiment and is taken to the headquarters of the operation, we barely get to see the mastermind behind it. His screen time is so short-lived, that I wonder why the writers even decided to put him in the story. And plus, the way Mike killed everyone at the end was just so random and super unconvincing.
And by the way, that very last scene at the end saying "phase 2", reminded me of the end to The Killing Room.