http://apps.leg.wa.gov/Rcw/default.aspx?cite=9A.16.040

In case you haven't read it, this is the RCW that Satterberg is trying to protect Birk under. Pay close attention to (3).
(3) A public officer or peace officer shall not be held criminally liable for using deadly force without malice and with a good faith belief that such act is justifiable pursuant to this section.

Without malice and in good faith belief.

http://legal-dictionary.thefreedictionary.com/malice

Legal definition of "malice"
Again, section (3)
3. It is a general rule that when a man commits an act, unaccompanied by any circumstance justifying its commission, the law presumes he has acted advisedly and with an intent to produce the consequences which have ensued. 3 M. & S. 15; Foster, 255; 1 Hale, P. C. 455; 1 East, P. C. 223 to 232, and 340; Russ. & Ry. 207; 1 Moody, C. C. 263; 4 Bl. Com. 198; 15 Vin. Ab. 506; Yelv. 105 a; Bac. Ab. Murder and Homicide, C 2. Malice aforethought is deliberate premeditation. Vide Aforethought.

The RCW does not give a peace officer a right to shoot whoever he pleases without a reprimand and he can be held criminally liable for murder given that the he has acted in malice AND was not acting with "good faith belief"
I believe he is, at the least, subject to a manslaughter charge. But given that he had his gun drawn, that he did not identify himself as an officer before he discharged his weapon and that he shot him in the back and side according to autopsy, it would appear that he acted without "good faith belief" and with malicious intent.

http://www.youtube.com/watch?v=D1VKo6-m27c

The video shows clearly Williams holding a block of wood that he was concentrating on while he crossed the street. Never in my life have I heard of someone killing their victim with a 3 inch carving knife, 10 feet away while they had their back towards them. Nobody could have thought by any measure that Williams was a threat and you can hear an eyewitness claiming nearly immediately that Williams had done nothing. The inquest has a very limited number of witnesses available, which is why you see the mixed results of the inquest jury. More evidence would be available in a criminal prosecution.

Satterberg's claim that he cannot prosecute is BS, and his claim that it would be fruitless only amplifies the public image of LEO's being above the law. Satterberg should have filed charges and let the jury decide, but instead, he decided to imagine himself as the jury and circumvent due process.