Video evidence is of little use when the people in charge turn a blind eye to it, are culpable themselves, or denounce it as "fake news".

When the have the cooperation and participation of the military, the secret police, and the general population (as has often been the case in many atrocities, genocides, and pogroms in the past) it's even worse, and by the time the video evidence gets out (if it ever does) it's too late to save the victims, assuming there's even anyone in the position to do so.