The video appears to be damning: a white police officer fatally shooting a black man in the back as he ran away. The shooting of Walter Scott by North Charleston, S.C., police Officer Thomas Stager has generated national outrage. Stager has been fired and charged with murder. Legal and police tactics experts said the video would be powerful evidence in the criminal case. But they say it still leaves Stager’s defense with options. One likely defense focus will be what is not on the video. The interaction between Scott and Stager before the shooting is not seen, nor is the...