It's not as black and white or simplistic as that, no
But in this case an inquest has been held in front of a jury and they decided,
after seeing and hearing all the evidence, that Ian Tomlinson was unlawfully killed.
So then the case is passed to the CPS, who after deliberation have decided there is now enough evidence to bring a manslaughter case.
http://www.cps.gov.uk/news/press_statements/cps_statement_on_mr_ian_tomlinson_/
So, he still may not be convicted, that will be for another jury to decide.
But if he is, no matter how outraged people are on here that the policeman is being prosecuted at all, he is being given the full and fair process of the law.
imo - as it wasn't just a push, but a baton strike then a push to the back against a person with their hands in their pockets walking away from the line, then that action was unreasonable.
No matter how difficult the job of crowd control is/was that day, no matter how much abuse the policeman got off other troublemakers - he is trained to be a professional and act accordingly and within the law.
A jury may find he didn't