The point of argument I believe is not whether Mr. Tomlinson was assaulted by the officer - you'd have to have a pretty brass neck to pretend otherwise - but whether any injury he may have received as part of that assault was the cause of his death. Remember murder & manslaughter have to be proved 'beyond reasonable doubt'; if you can't prove the officer's actions directly led to his death, you're not going to win against a reasonable defense lawyer.
The officer in question should certainly be sacked and should have been prosecuted for assault and/or ABH, crimes which would have been readily proveable. Unfortunately whoever was running the prosecution chose to go for the more serious offense and, to put it bluntly, they blew it.
Of course, if you knew that a charge of murder/manslaughter probably wouldn't stick and you wanted the officer to get off, that's exactly what you'd do too....
Overall, this will only have made the rapidly souring relationship between the public and the police worse. A pretty awful day for Mr. Tomlinson's family but not a good day for the rest of us either.
Jon
