What if: Active Shooter

I heard the gunshots and thought to myself for the hundredth time "God, I hate the city." a Canadian used to wide open spaces I had already had enough big city to last a life time and now this. Eden and the kids were five floors down in a toy store and best as I could tell the gunshots were coming from my floor. I know they were gunshots, I'd spent enough time an the range to recognize them even before the screams started.
A quick recon showed a lone gunman about a hundred meters out heading towards the Oakley store where I had found myself pricing out overpriced sunglasses I would buy a knockoff of from a street vendor later. My security instincts immediately kicked in and I immediately escorted everyone out the back door into the maintenance tunnels, once everyone was safe I went back to the entrance and peek out between two displays. The gunman had a pump action shotty and was now fifteen meters from the entrance, I ducked back down, set a timer on my phone for fifteen seconds and slid it to the opposite corner of the store.
Being Canadian I wasn't allowed to bring guns into the states but I had spent years bringing knives into places I wasn't supposed to, today was no exception. My right hand pulled the Pentagon mini dagger from the small of my back while my left index finger hooked through the Snarl dangling from my neck.
Footsteps close now, the racking of a slide, the tinkling of a discharged shell. Adrenaline had completely taken over by now, my fight or flight stuck permanently on fight. I knew one thing and one thing only, this person had harmed innocents and as long as he was free to continue his rampage my family was in danger.
The phone goes off. A black shotgun barrel snakes through the door. The owner follows, his attention on the sound.
No time to think, just act. I lunge upwards the dagger stabs out for the man's throat, the Snarl slashes at his trigger finger. Red arterial blood follows the dagger as I pull it back.
The shotgun clatters to the stone floor, I kick it away but it doesn't matter.
The gunman collapses to the floor, doing his best to stem the flow of his life force. My adrenaline flow stops and I suddenly feel very tired. I sit down, nothing to do now but wait for the Calvary to come arrest me.