How do you shoot someone in the back of the head in self defense?

Paul Kennedy, chair of the commission for public complaints against the RCMP has concluded that Constable Paul Koester's story that with his face down in a couch cushion and a six foot 187 pound Ian Bush on his back throttling him, he was able to draw his weapon, reach behind his back, behind Ian Bush and shoot him in the back of the head is the truth.

Koester, who stands 6-4 and weighs 180 pounds, insisted the six-foot, 187-pound laborer was atop his back choking the life out of him when he managed to free his gun. In a physical feat even RCMP investigators conceded was worthy of a contortionist, the Constable got the gun behind his own back, up to the back of Bush's head and shot him. He refused to reenact what happened for investigators and the coroner's inquest that was held earlier this year.

The RCMP seem mystified as to why anyone could have any doubts in this version of events, firmly believe they have no image problem with the public and indignantly reject Kennedy's call for video and audio equipment in all areas where prisoners are dealt with as 'unnecessary'.

UPDATE: Gary Mason, behind a paywall, flatly says he doesn't believe the RCMP and Constable Koester's description of the death of Ian Bush:

Like many others, I don't believe his version of events. I just don't see how he could have hit Mr. Bush three times in the back of the head with the tip of his gun and then shot him, all the while being face down on a couch and with Mr. Bush lying on top of him.
The fact that Constable Koester refused on the advice of his lawyer to be part of a re-enactment at the inquest into the shooting only confirmed my doubts. Personally, I believe Constable Koester was on top of Mr. Bush, hit him three times in the back of the head with the tip of his gun and then hit him a fourth time when his gun accidentally went off.

However, in a demonstration of institutional schizophrenia, the Globe's editorial board disregards the opinion of the reporter covering the story since the beginning. They accept the RCMP version unreservedly. They call the Kennedy report that Mason dismisses as fatally compromised 'persuasive' and claim Constable Koester is 'entitled to respect and a fresh start.'

I submit that Constable Koester's fresh start should be in the neighborhood lived in by the Globe and Mail's editors dealing with their children in unmonitored RCMP backrooms.