Bad things happen when good people stand by and do nothing. Confronting those who are wrong with the truth and maybe changing the way they think is better than waiting for them to get radicalized and start killing again.
And who are you to say they are wrong and that you know the truth?
Who are you to make anyone change how they want to think and do with their lives?
How is having a concert a bad thing, and when did you become all knowing and able to see into the future as to how things are going to happen?
From what I have seen and from what I know of such groups, you are not going to have any input in to changing their way of thinking, other than causing trouble and being a threat to them, forcing them to defend themselves.
And how then will that change them? other than prove to them they are right in their thinking.
Why not leave them alone and let them have their time to themselves.