The way I see it is that Officer Wilson was just performing his job, ably and admirably. There was not a scintilla of wrongdoing and thus it was unjust to present it to a Grand Jury at all. There simply was no case and the ‘RAT McCullouch is the guilty party. Wilson’s good name is irrevocably tarnished because of ‘RAT misdeeds.
I dunno if you understand the purpose of grand jury.
A grand jury is a legal body that is empowered to conduct official proceedings to investigate potential criminal conduct and to determine whether criminal charges should be brought. A grand jury may compel the production of documents and may compel the sworn testimony of witnesses to appear before it.
Had Florida vs George Zimmerman did grand jury they'd not wasted effort to prosecute Zimmerman.