Just supposing Mr Cooke told the police to P*** O** he would then have had to be arrested. As for cautioning him, as soon as police have evidence that a person MAY have committed an offence they MUST caution them, whether they have been arrested or not.
This case had to be investigated, without that no one could prove if a crime had or had not been committed.
As we know from the Tony Martin case (the Norfolk Framer) if unnecessary force is used it amounts to a criminal offence. Just because a person who has a criminal record is committing a crime, that does not justify him/her being killed.