People are afraid of the unknown, so it's easy to read something in the news and then apply that, in this case, to the whole event/festival.
The actual reality from someone that's been a handful of times is that it's perfectly fine other than it's stupidly busy, which may not appeal to those that struggle with crowds, and i was more worried about pickpocketing (no worse that roaming through Piccadilly though) than getting shot or stabbed.
Similar has happened at other festivals though - someone got stabbed at reading a few years back, same with Creamfields and Global Gathering.
I'm surprised you don't venture further than your front door given there's been a few incidents in your neck of town over the years