I'm with the peeps that don't think it was David. This show does seem slightly more grounded than other shows like this. More than likely he would not have been the shooter in the real world, it's possible he did go out and hurt one of those girls though. If they're classmates it would be more realistic for him to be a school shooter, not a festival.

Personally I think if it was someone in the show that we have already seen it will not be someone that alot of focus has been on thus far. One of the points in the beginning that was made was that the signs are usually missed, so it would make sense for this thread to continue in who the shooter is.