You could argue that it was a lethal force situation the moment that guy picked up a knife.
Ability? Check
Opportunity? Check
Jeopardy/intent? Not exactly clear, but picking up a knife after a cop told you to drop it isn’t exactly a show of good will. I’d call this a check too.
We can wonder just how stupid that guy is and hope he doesn’t go on to reproduce, but the reality is he could have been on the cop with that knife in about one second and the cop had no idea what he was going to do.
Maybe the taser stops him, maybe it doesn’t. Only one way to know how it would play out, and luckily it wasn’t learned that day.
Obviously the cop in the video showed a lot of restraint and the outcome was good. Contrast that with the video I linked way above, and restraint put an officer’s life in grave and immediate danger.
I don’t think most cops are eager to shoot anyone. Both videos demonstrated that. Few people are. I only encountered one borderline lethal force situation as a bouncer, but I wasn’t particularly eager to hurt anyone on the job when handling violence with lower stakes. I took risks that I didn’t need to, legally or morally, in order to respond with the least amount of violence.
I’m also stronger than most LEO’s with much more hand-to-hand training than most LEO’s. I’m not some ultimate badass, but I recognize I have an edge most cops don’t in a bare-handed struggle. I KNOW how hard it can be to subdue someone, even with a huge size/strength disparity, a skill disparity and the edge of sobriety.
People have this image in their head of cops moving like they’re in a Steven Seagal movie, disarming knives and subduing violent people with no harm coming to either of them. That shit doesn’t happen without some combination of luck and/or lots of training on their own time and own dime.
To top it all off, as @Frank_C pointed out, a gun is involved in EVERY struggle a cop gets into.