Petty motvations ...

... may well lead to something else.
Remember the case of the fugitive police sergeant? (The first item in this post) Now he's no longer a fugitive.
Now the whole thing seems to be less dramatic than it was first made out to be. Or actually, no, that's not too true. Only now the picture is more complete, and we are made aware of the possible role of the personal, more banal, motivations of the main characters involved. Thus Yang may have been encouraged to report the misdeeds of his superior as much to gain an one-up on him as to uphold justice, and his superior to retaliate more in response in this game of one-upsmanship than for motives dark and sinister.
Of course, in the final analysis, the misdeeds perpetuated by the superior might well lead to misery and suffering -- or less dramatically more petty lawlessness. And the public controversy over the Diary published online may, hopefully, lead to increased scrutiny of and transparency in the workings of the Chinese police. If the current outcry over melamine-tainted milk powder does not compeltely eclipse the subject, that is. Doesn't the whole episode remind you of Thomas Carlyle's dictum that history is mostly driven by individual actions? Or Hannah Arendt's banality of evil? Petty motivations drive individual actions which interact to produce significant social consequences and phenomena.

(Admission: both ideas got slightly twisted in the last sentence)