The decision not to prosecute the police officer who killed Ian Tomlinson isn't just wrong - its is also dangerous. Here's why.
When it comes down to it, I am some form of Hobbesean. No, not his absolutism, or his idea that there was an actual contract, but his core idea (which is the foundation of most modern political theory) that government exists by consent and that its primary purpose is to save us from the state of nature. One of the ways it does that is by protecting people and arbitrating disputes. Locke refines on this and basically says that government has to deliver justice; while he shrouds this in superstitious nonsense about "implementing the law of god", the cynical reason is more compelling: because otherwise people take matters into their own hands.
When the government's justice is so transparently biased, it raises the risk that people will seek their own justice. Now, I don't really think the aggrieved family of Ian Tomlinson are going to get a lynchmob together (and sadly, they can't afford to mount the private prosecution this case deserves) - but the fact that that looks like a more just solution than what the government is offering fundamentally undermines the government's legitimacy, and its a sad commentary on the state of the British government today.