The specific role is someone who lethally attacks anyone who does a night action, House rules advise limit one 'customer' per night though that needn't be so though I also doubt the different modes of death - stab, shot, and strangled - indicate our theoretical PGO didn't pull a double let alone a hat trick

PGO though, at least as I use it, is generally just any role which lethally counter-strikes against someone who does something besides just an NK attempt. Like a one-shot counter-strike or a 50% to counter-strike or strikes down investigators, RBers and NKs but not Docs, etc.