They've been in a tight place, or they've wanted something very badly, money or a woman - and they've killed to get it.
The brake that operates with most of us doesn't operate with them.
A child, you know, translates desire into action without compunction.
A child is angry with its kitten, says
'I'll kill you,' and hits it on the head with a hammer - and then breaks its heart because the kitten doesn't come alive again!
Lots of kids try to take a baby out of a pram and 'drown it,' because it usurps attention - or interferes with their pleasures.
They get - very early - to a stage when they know that that is 'wrong' - that is, that it will be punished.
Later, they get to feel that it is wrong.
But some people, I suspect, remain morally immature.
They continue to be aware that murder is wrong, but they do not feel it.
I don't think, in my experience, that any murderer has really felt remorse...
And that, perhaps, is the mark of Cain.
Murderers are set apart, they are 'different' - murder is wrong - but not for them - for them it is necessary - the victim has 'asked for it,' it was 'the only way.'"
"Do you think," I asked, "that if someone hated old Leonides, had hated him, say, for a very long time, that that would be a reason?"
"Pure hate?
Very unlikely, I should say." My father looked at me curiously. "When you say hate, I presume you mean dislike carried to excess.
A jealous hate is different - that rises out of affection and frustration.
Constance Kent, everybody said, was very fond of the baby brother she killed.
But she wanted, one supposes, the attention and the love that was bestowed on him.
I think people more often kill those they love, than those they hate.
Possibly because only the people you love can really make life unendurable to you.
"But all this doesn't help you much, does it?" he went on.
"What you want, if I read you correctly, is some token, some universal sign that will help you to pick out a murderer from a household of apparently normal and pleasant people?"
"Yes, that's it."
"Is there a common denominator?
I wonder.
You know," he paused in thought, "if there is, I should be inclined to say it is vanity."
"Vanity?"
"Yes, I've never met a murderer who wasn't vain... It's their vanity that leads to their undoing, nine times out of ten.
They may be frightened of being caught, but they can't help strutting and boasting and usually they're sure they've been far too clever to be caught."
He added: "And here's another thing, a murderer wants to talk."
"To talk?"
"Yes, you see, having committed a murder puts you in a position of great loneliness.
You'd like to tell somebody all about it - and you never can.
And that makes you want to all the more.
And so - if you can't talk about how you did it, you can at least talk about the murder itself - discuss it, advance theories - go over it.
"If I were you, Charles, I should look out for that.
Go down there again, mix with them all, get them to talk.
Of course it won't be plain sailing.
Guilty or innocent, they'll be glad of the chance to talk to a stranger, because they can say things to you that they couldn't say to each other.
But it's possible, I think, that you might spot a difference.
A person who has something to hide can't really afford to talk at all.
The blokes knew that in Intelligence during the war.
If you were captured, your name, rank and unit but nothing more.
People who attempt to give false information nearly always slip up.
Get that household talking, Charles, and watch out for a slip or for some flash of self revelation."
I told him then about what Sophia had said about the ruthlessness in the family - the different kinds of ruthlessness.
He was interested.
"Yes," he said. "Your young woman has got something there. Most families have got a defect, a chink in the armour.
Most people can deal with one weakness - but they mightn't be able to deal with two weaknesses of a different kind.