One guy said that he saw the woman next to him shot in the head, "If only I'd had my gun...".

i wonder how 'having his gun' would have been of any benefit to the dead woman? How would he have managed to 'take out' a distant assailant he couldn't see and didn't know for sure where he was?

If people had 'had their guns' at the festival, the result would have been considerably more people dead and injured. Anyone with at least half a functioning brain should be able to understand that.