I watched it a few times but you can’t see who the killer is.
All I see are people moving in front of the woman. I don't see her being shot, though I hear a surprisingly quiet gunshot before she is lowered to the floor. Comments at the site say police did it, but I don't see that. There is a cop behind her with a rifle, but it doesn't appear to me as though he just used it. Some Freepers will know whether or not the sound of the gunshot could have been made by that rifle.
Some say it was the cop on the stairs, up above the crowd.