what drunks police chose to arrest in public

My question is a legitimate one. What drunks police choose to arrest in public? Should police be liable when the drunk get behind the wheel and drive cause a accident. Recently four teenagers were killed by a drunk driver that police had encountered less than two hours before the accident took place. Ok, so the police admitted that the woman wasn't breaking any laws. But the man house she was at called the police complaining that the woman had broken his window, mailbox, being violent and drinking heavily. The police warned her not to drive. If the woman was drinking and her alcohol blood level was three times the legal limit, was she coherent enough to understand what they were saying? Who's the blame here for the loss lives of the innocent children that was killed?