Not intervening is clearly the right reaction if you have no realistic chance of overwhelming the attacker.
I disagree. The "right reaction if you have no realistic chance of overwhelming an attacker" is to prepare. The missing first step in preparation is that the person available to intervene had never taken the time to get their head around circumstances where they would be willing to kill somebody, so they weren't prepared. I won't claim to be prescient, but I have that list pretty well spelled out.
As to "unarmed"...I'm
never armed just walking around. There are
always weapons of opportunity, and I think
everyone should practice not only knowing when they are or aren't willing to act, but recognizing what is at hand moment to moment to make such action possible...or at least more probable in terms of success. I'm sitting at my usual seat at my computer desk in my gf's dining room. There are big effing knives six feet to my right in the kitchen. I can reach them over the counter and don't have to go around into the kitchen, and I know that because I've checked. There's a dog crate next to me, with an empty laptop case on top of it. Under that case are two two pound dumbbells like you would use in an aerobics class. I didn't intentionally hide them from view, but when I dropped the laptop case on top of them I did make a mental note they are there in case I ever feel a need for them, because with a two pound dumbbell in hand even a moderate puncher can kill someone handily. The dining room chairs are too awkward to get out from under the table in a hurry due to limited space. There's a fireplace poker about ten feet to my left. I'd have to go over the back of the couch to get to it from here, but if I were in the living room it might be my first choice. One of the table lamps has the cord running under a recliner that would likely get snagged if I grabbed that lamp, but the other got the plug chewed off by a dog several years ago and the cord is coiled under the lamp so it wouldn't have any chance of snagging. If I wanted to grab and throw that lamp, or shield myself with it I would need two hands, and it wouldn't be an effective club, which I know from considering it while handling that lamp. The recliner could be kicked over from either front or back to create an obstacle, the couch can't. There's a pen in a jar on my desk, and pencils in a holder on the side of the refrigerator...and I've seen first hand how effective a pencil in the neck can be. The candle holders on the mantle could have been
designed as weapons and they wouldn't be easier to grab and throw.
Now, admittedly that's a room where I spend a lot of my time. I don't inventory every space I enter that thoroughly. But I can honestly say that I never enter any space without inventorying
at all. A pilot friend of mine tells me that he
never doesn't know where every strip within gliding distance is, even though he has never experienced an engine failure and the odds are very good that he never will. I figure if he can prepare so can I.