For the entire length of the film ATM, the killer remains maddeningly outside of the ATM kiosk where he has trapped three yuppies on a supposedly-cold Christmas night. For most of the film this is a puzzling decision, as it seems to be born of his desire to avoid being caught on a security camera. Which is normally an understandable thing for criminals to avoid, but in this case it doesn't make the most sense, as the killer's wearing both a hooded parka and a ski mask, ensuring that his identity is well-secured.
Finally the meaning behind his baffling decision is revealed when the film asks us to believe that the killer could have successfully framed one of his surviving victims for all of his crimes. Which include three murders, a negligent homicide (at least), and extensive acts of vandalism. The film implies that the killer will get away with this solely because he never actually appears on the ATM's security camera footage.
So, what would happen the next day?
Examining the footage and listening to the survivor's story, the police would conclude that yes, they're obviously looking at video of three people who are besieged by a maniac who trapped them with a car and then tried to cause them to freeze to death by flooding the kiosk with water from a nearby firehose. There's nothing in the tapes that supports any other interpretation of the evidence, especially because the three victims are constantly looking and yelling out the windows, desperately trying to figure out what's happening to them. Not to mention the fact that none of the implements used in the film's murders have fingerprints on them, since the killer - unlike any of the three main victims - was wearing gloves.
Which would a cop more likely believe - that an unknown madman killed a bunch of people at an ATM, or that some random banker hung out in an ATM kiosk for three hours with his co-workers for no reason, then tried to flood the place before driving two separate cars into it?
Yes, the survivor would be on the hook for that janitor that the two of them killed, but given the situation (a man dressed like a murderer barged into an ATM kiosk without warning, then said nothing in his defense while being attacked in response) the most he could realistically be charged with is involuntary manslaughter, and he likely wouldn't serve any time (assuming no prior convictions).