Your mind was on something else just a few seconds and a lifetime ago.
Something enjoyable. Something fun. You were in the moment just a second ago. The music was pulsing, the lights were throbbing. Everyone around you was jumping up down, their faces grinning wide with enjoyment.
Your mind wants to get back to that place, wants to quickly explain away what is suddenly going on around you. The screams are little different but their was plenty of screaming a moment ago and it was alright then. Everything is suddenly a little bit distant. A little bit disconnected from the reality of what is happening around you.
You are in Condition White and the reflexive state of Denial you are in, has a good chance of getting you killed.
You are metaphorically speaking on top of a tower that is on fire and the life saving rope in front of you is just inches away from the tips of your fingers. You can grab it but only if you lean forward and commit.
First, get a hold of yourself.
That is job one.
Next. get a hold of your Goto Guy.
Maybe your Goto Guy is your wife. But in this situation it maybe your son. What you need is level headed in a crisis. So make sure you know who your Goto Guy is before the SHTF.
Don't listen to the orders of terrorists telling you to get down on the ground. It may seem obvious to you but the audience of the Bataclan largely died because they either obeyed terrorists orders or simply stood there in shock.
Should you fight?
This is the point where the keyboard commadoes will start going on about how they will kill all those muthfucking terrorists if they were there.
Yeah not so much.
If you are armed it's probably with a pistol. Most likely a subcompact. But even if you have a full size pistol on you, you need to remember Cataline's Golden Rule: a pistol is only useful for fighting your way back to wherever the hell you've left your rifle. If the OPFOR is armed with a rifle your odds of survival are bad. The basics are what they are, a rifle is intrinsically more accurate and fires a more powerful round. Likely your OPFOR has a lot more of those rounds both in his weapon and on his rig. Possibly he's armored. These are all very long odds for a lone pistolero.
Your second problem is legal.
Back when I was in and spun up on this stuff. I would, given my high level of training, rated myself at a B- for engaging a terrorist and not splashing any collateral next to him. Truth be told, this is very, very tricky stuff. There are nothing but variables. To include a constant stream of idiots jumping into your line of fire. Your target will be surrounded by potential collateral anyway. And as I said before you are firing with an inaccurate platform in the first place.
If a cop or a solider accidentally kills someone they have legal protection. You as a private citizen have absolutely none whatsoever. In this political environment the likelihood of you being charged with second degree murder is near to 100%.
Now all of that said, if your terrorist opens up five feet away from you, then you are on Death Ground. If you are on Death Ground FIGHT. Annnnnnnd if the terrorist opens up at contact range with his back towards you...hey, you aren't responsible for his over sight. Tango Down.
In this day and age if you are going to go to a concert or any public venue have a plan for getting out of it.
Be familiar with the venue and consider it tactically. Select your seat with some care. Do not go with Festival Seating. Are there nice big pillars that can be used as cover? How tall are the seats? They won't stop a bullet but the terrorist can't see through them either, that makes aiming problematic. Which doorway gets you outside the building fastest? When you first arrive and your family settles in, scout out your primary, secondary and tertiary exfiltration routes.
Lastly, some advice if you have to shelter in place. For preference pick a room with a door that locks and is an absolute bear to get through. You can find those more easily in a school than in an arena but it might happen. Get out your smartphone phone dial 911 and start getting intel out to the cops. Tell them where you are and especially if you are armed.
If you can't find a room like the above then find one where you can set up an ambush. As a prime example; most stadium lavatories in the US have an open door with a solid (often concrete) wall acting as a privacy curtain. In tactical terms the terrorist will be approaching you at a blind dog leg turn and one where he can't bring his rifle to bear before you can have your pistol will be in a firing position. Corners are the one place...indeed the only place where a pistol is preferable to a rifle. This is also the only situation where I would recommend going for a headshot. A neurologic is the only way you can be sure to put him on the ground. You are at contact range...go for it.
The truth is that your odds of being eaten by bears is way better than your odds of being caught in a terrorist mass shooting.
But they aren't zero...plan ahead. Your survival is on you.