"People have been killed," I said, and could have bitten my tongue out. Why let him even suspect that I was thinking about something that wasn't tied in with his argument at all, why give him the slightest hint? The Underground's accident record was good and couldn't have justified such cynicism on my part. And just suppose he wasn't the garrulous, middle-aged business man he appeared to be-- A very sinister game can start in just that way, with everything favoring the alerted party until he lets the other know that he's on his guard and is having uneasy thoughts. That's where the danger lies, in a subconscious betrayal, a slip of the tongue that will precipitate violence faster than it would ordinarily occur. If a killer feels that he must move swiftly, before suspicion can become a certainty, the odds shift in his favor. He has the advantage of surprise. He becomes alerted too, and necessity acts as a goad--a kind of trigger-mechanism. He'll act more quickly and decisively, without the careful planning that may prompt him to talk too much and give himself away. He'll take risks that are dangerous and could destroy him, strike with witnesses present and all escape routes blocked. If he has to, he'll strike even in a crowded Underground train with the next station minutes away. And that kind of audacity sometimes pays off. I told myself that I was imagining things, jumping to a completely unwarranted conclusion. The conversation of the man next to me was exactly what you'd expect from a magpie. He was carefully sidestepping all realistic appraisals of the Underground's shortcomings, trying his best to look at the problem from all sides, even if it meant being shallow and over-optimistic. He was the citizen with a smiling face, the rather likeable guy--why should one hold it against him?--who was trying his best to be fair to everybody, even if he had to burst a blood-vessel doing it. Realizing all that made me feel less tense and part of the nightmare feeling I'd been experiencing went away. But not quite all of it and when the train passed into an unlighted tunnel and the aisle went dark apprehension began to mount in me again. What if he was putting on an act, and wasn't the kind of man he appeared to be at all? What does a killer look like? Certainly age had nothing to do with it. He can be young or old--eighteen or seventy-five. His appearance, his clothes? There were wild-eyed killers with "psycho" stamped all over them, and dignified, soberly-dressed men who looked no different from your next door neighbor and had criminal records a yard long, including, in all likelihood, a murder or two the Law would have a difficult time proving. I didn't have to speculate about it. I _knew_, because I'd done more than my share of social research. There was nothing to prevent a man of distinction from becoming a killer, if he had a secret life that was ugly and devious and a powerful enough motive.