The entire film seemed pointless and avoidable. There's 3 of them and 1 of him, and he already proved that he was unarmed in the beginning. Yet the characters continue to quiver in fear and do nothing but acknowledge the numbers game. That had to be the most irritating part of the film. Our antagonist has issues, just watches the protagonists and never speaks. We really don't see his face either. While the film wasn't boring, it certainly wasn't good either.
ATM should have been great. The convoluted, but ingenious plan of the antagonist was quite interesting. However, the illogical decisions made by the trapped people was crazy. They should never have been trapped for that long. There were three of them, they could easily have taken down the assailant.
Anyway, ATM has a lot of plot holes, far too many.