Breaking in into a bank through the ATM machine is probably the worst idea ever. Banks (or at least the banks I worked at) have a motion detector in the room behind the ATM. Only once I saw a bank that had an ATM removed and just covered up with plywood from the outside while the motion detector was disabled in that room. Triggering the ATM alarm is worse than the premises alarm because the premises alarm gets triggered sometimes from cleaning personnel or other employees but for the ATM room you need a special key that not everyone has.
I'm also not sure that you can easily go into debug mode without anyone noticing (assuming some employee let you in that room) because the ATM technicians have to call Diebold before doing anything with the machine. They will know if someone unauthorized is using the ATM and restarting with a live CD won't work because that will also trigger an alarm.
I'm guessing it was an Diebold employee that installed the malware since he would have been the only who could have gotten that much access to it.