I've watched the video at least 5 times. You say the guy reached for his gun, with both arms pinned? I don't see that, and neither will any jury.
You may be right. It was hard for me to see what his arms were doing. The bottom line is that the officer seemed to use deadly force unnecessarily.
He didn’t have both arms pinned, though.
I watched the video in slow motion and captured some screen shots. See my comment here:
http://www.freerepublic.com/focus/f-news/3446942/posts#60
His left arm was pinned, but his right was apparently not, and it appears he might have reached for his gun and that he and the officer on top of him were struggling for it for several seconds before the officer shot him.
Also keep in mind that while they likely had the report that he had a gun and threatened people with it, they didn’t actually pull their guns until one of them found that he had one. One threw him on the ground and they tried to subdue him physically, without their guns drawn, until they apparently discovered he had one.
We don’t know why, either, they didn’t seem to treat him as armed until that point, although apparently the call to police said he was.
At this point there are really many unanswered questions, but the video seems to suggest that Sterling was only partially subdued, his right arm was free and he struggled with officers with it, and while it is yet to be shown either way, that might have been the hand that would have had access to his gun.