It's not about a particular event. It is just saying how ironic it is that some people criticize their music because they say their concerts are violent and dangerous, while the people criticizing them do things much worse than what goes on at a DM concert. In short, they are mocking their critics by actually singing about something very violent, but they put their message across in the lyrics that there are other things to worry about than a mosh pit.
It's not about a particular event. It is just saying how ironic it is that some people criticize their music because they say their concerts are violent and dangerous, while the people criticizing them do things much worse than what goes on at a DM concert. In short, they are mocking their critics by actually singing about something very violent, but they put their message across in the lyrics that there are other things to worry about than a mosh pit.