The Punisher is a villian who suffers from a vanished conscience. Too many times , we, as people, wish to exact revenge instead of justice to solve our dilemas in society. Some of the "scum" he executes may have been better off being apprehended. This in turn could cause them to flip on their associates or possibly even be rehabilitated. The executions that the Punisher doles out eliminates those possibilities and probably hampers the law enforcement officers who are embroiled in investigative operations. This closes doors to the officers and may even allow bigger fish to get away. Further, who is to say that Frank Castle, without having total access to the undercover work done by the various branches of law enforcement hasn't killed an undercover officer by mistake. Of course the writers would never allow that to have happened, it is of course a possibility, As for the little girl he almost killed, that alone should have caused him to give up his lethal vengence since his actions of war on crime would at some point have to have collateral damage. The Punisher is a danger not just to criminals but to society as a whole. Eventually as a "role model" for taking the law into our own hands, who then will decide on what type of crime deserves execution and worse, what if the wrong person is executed.
The Punisher is a villain.