From Wikipedia: "Dialectical materialism is a strand of Marxism, synthesizing Hegel's dialectics, which proposes that every economic order grows to a state of maximum efficiency, while simultaneously developing internal contradictions and weaknesses that contribute to its systemic decay."
So, basically, the fact that Simon Harwood has been found innocent, along with the fact that the police have a long history of protecting their own as I said previously, this shows an example of dialectical materialism, whereby we have a relatively advanced, well off, forward-thinking society, but internal corruption such as the Harwood example is a sign of an internal decay of that very society.