I teach Active Shooter response. The odds you'll be involved in an AS incident are infinitesimally small. The odds you'll be involved in a way you might envision, are exponentially smaller. You simply won't know what's happening and what the best course of action is until you're deep into it. Even the most alert of us don't spend all our time in public scanning for the next big threat. The odds of knowing what's about to happen and what to do are orders of magnitude smaller than the aforementioned.
So what should you do?
Interviews with survivors of critical incidents reveal two types. The first type is just pure dumb luck. They don't really plan anything and don't react with any purpose or speed. They just exist in the precise bubble that allows them to live.
The second type positions themselves to survive through both conscious and subconscious skill. They walk into a space and observe where the exits are. They select where they will stand or sit based on their ability to observe who comes and goes. They scan crowds and automatically create space from those who make them uneasy. They think about what they'd do in the event a crisis emerges. They pre-pay for the most critical element in a survival scenario, time.
Based on your awareness, skillset and tools at hand, you should trust your instincts and follow them without hesitation. If you're in close and you react quickly enough and with every bit of violence your mind and body can muster, you might just come out on top. Once you commit to an attack, commit beyond all reason. Allow millions of years of primal instinct to come out. All. Out. I mean be savage, howl, rush, kick, hit, scream, envision your hands crushing his skull against the cinder block wall or on the floor, snapping his limbs with your knees, dislocating his elbows and shoulders by hyperextending them beyond all reason. Do not stop until there's nothing but a bloody puddle of flesh and pulverized bones. Make them pry you off of his gelatinous corpse.
If you're not in immediate proximity, your mind needs to be prioritize responses from most to least immediate. Where is the threat; where is concealment; where is cover; where is safety; where is escape; where are weapons; where are reinforcements; where are 1st Aid supplies and so on and so forth.
If you're in a group, you need to become an instant battlefield commander. You give directions with as much force of will as necessary to ensure they follow them without hesitation. Once cover is gained, you give orders by looking directly into people eyes and telling them exactly what to do. "You, barricade the door with these desks."; You, grab that fire extinguisher and bring it to me."; "You, look for other things that could be used as weapons."; "You, call 911 and tell them the shooter is in the West wing of the science building."; "You, Tweet the active shooter alarm over school/company social media."
Even someone too frightened to do anything at all can be a useful tool. You might place them in such a way as to distract an active shooter coming through the door, from a "stack" of people with improvised weapons waiting to ambush him. Unfortunately, every battle needs cannon fodder.
Actually visualize yourself doing these things in your daily environment. Train for it and your odds of survival jump exponentially.
Now if you're carrying, the immediate proximity response should be to shoot that fvcker in the face. Man or beast, if you shoot it in the face, it will probably leave you alone. If you're beyond visual range, you have a crucial decision to make. How close are you? How likely are you to shoot the wrong person? How likely are you to arrive at the same time an armed guard or sworn officer will arrive? How will you avoid being mistaken as one of the active shooters?
Stats overwhelmingly show that most active shooters self terminate the instant they meet effective resistance, so if you can force him to self-terminate, by all means do so. FWIW, unless I see a glorious fountain of blood and defective gray matter, anchor shots are on my personal agenda, followed by immediately re-concealing my weapon and barking orders like a battlefield general. While acting as temporary on-scene commander, make sure you keep both hands open and slightly away from your body. When the first responding officer arrives, you want to be the least threatening thing this side of a cute fluffy bunny. Follow their commands immediately and accurately. Now isn't the time to convince the adrenaline fueled officer that you're the good guy.
Now let's say you work in an office and don't want to get fired for bringing a gun to work. Do you have walls? Can you hang a picture? If so, bring a picture, hang it on the wall and then throw this in your bottom desk drawer:
That's a Dewalt Mig Weld 15oz Framing Hammer and it's the Lamborghini Aventador of hammers. You could cleave skulls all day long with those high tensile strength straight claws, all while the anti-vibration handle prevents hot spots and sore muscles!
That's what I teach. You can scan the audience and see exactly who the cannon fodder are. They get this queasy, squeamish look and squirm in their seats when you mention violence and terminal effect. The ultimate lesson is "Don't be cannon fodder!" What ever you decide to do, do it 100% and don't look back.
Excellent advice.