There have been a bunch of conversations on here about how people don't react how they think they're going to react when they're in a life-threatening situation. Some run, some scream, but a big percentage either don't accept what's happening or freeze up completely.
I know when I got mugged, I froze. They were punching the crap out of my friends. My boyfriend tried to offer my purse as a decoy when they demanded his wallet, which was in his back pocket which he was sitting on. Rather than accepting that handing over my purse was smart, I started blathering about how there was no money in it because I didn't want to lose it. When they went to hit my boyfriend again was when my brain went into hysteric mode and I started shrieking for them to please stop and not hit us any more. This actually worked, and they backed off, reassured us they weren't going to hurt us any more, and got off at the next stop. They were probably less sympathetic to my tears and begging and more worried that my screaming and crying was going to attract attention. We waited two more stops before we contacted the guard for help, and he was visibly annoyed and confused as to why we hadn't come to him immediately. The truth is we were all too scared to move, even though the danger was long gone. Oh, and by the way, we were useless witnesses. We went to the police. When we talked later, all of our descriptions of the attackers were different. Ratios of race, age, number of attackers. We were dreadful, despite the fact that none of us could stop our heads echoing with afterimages of moments of an attack that probably took under two minutes. Between two stations. A distance of only a couple of kilometres.
We can have all kinds of ideas about how we're going to react. I was attacked on a train, by a group of teens like myself, who used nothing but fists and as far as I know were unarmed, with a guard in the next carriage, on a familiar section of line, in broad daylight. I have absolutely no idea how I would react if I was being attacked in the middle of the night in my home by a stranger with an enormous knife, and neither do any of you. The only folks who know are folks who've had it happen to them, and most of them don't survive to tell us. So we can't judge any of them, from the four that died, to D who saw the attacker, to the other roommate and the friends who came over the morning the attack was discovered. None of them.