If they killed someone, they would, at the least, be subjected to blood tests - that's the law - to determine if they were drunk or otherwise impaired. They'd also expect, at the least, a proseuction for reckless homidice, if not manslaughter. They'd also be sued, and hope that their insurance company defended. (And in a fatality, they'd be told that their insurance may not be enough to cover all the judgement.) All things that officers don't face. And my mistakes don't generally costs the taxpayers