BUT: Carry a camera or camcorder at all times. If someone near you is shot/you hear a shot/crack/whizz take a 360° pan of the surroundings even before you render aid.
If possible do two full pans, you MIGHT capture an image of the sniper. The double images will help highlight anyone bugging out...
However, bear in mind that a prudent alternative from the safety point of view would be to HIT THE DECK FAST and shout at others to do the same.
The first rule of first aid and rescue is not to become a victim oneself in the act of helping others (otherwise more and more people just get dragged into the situation). I am not sure that this emergency situation is sufficiently different from the norm to warrant putting evidence gathering above self preservation.
Also at the distances that the sniper is apparently working from, and the relatively low resolution of the typical camcorder (and digital and point-and-shoot camera), at a minimum the cost-benefit tradeoffs should be considered carefully.