They are four widely divergent types; the motive that drives each one of them to crime is peculiar to that person, and each one would employ a different method. The deduction must, therefore, be entirely psychological, but it is none the less interesting for that, because when all is said and done it is the mind of the murderer that is of supreme interest.

It was a soft purring voice a voice/used deliberately as an
inStrument–nothing impulsive or unpremeditated about it.
Hercule Poirot swung round.
He bowed.

The door of Mr. Shaitana’s flat opened noiselessly. A grey-haired butler drew it
back to let Poirot enter. He closed it equally noiselessly and deftly relieved the
guest of his overcoat and hat.

The man who came in did so with a kind of parody of a brisk bedside manner.
He was a cheerful, highly-coloured individual of middle age. Small twinkling eyes,
a touch of baldness, a tendency to embonpoint and a general air of well-scrubbed
and disinfected medical practitio/er. His manner was cheerful and confident. You
felt that his diagnosis would be correct and his treatments agreeable and
practical “a little champagne in convalescence perhaps.” A man of the world!

“In real life people don’t bother about being too subtle, Mrs. Oliver,” said the
superintendent. “They usually stick to arsenic because it’s nice and handy to get
hold of.”
“Nonsense,” said Mrs. Oliver. “That’s simply because there are lots of crimes
you people at Scotland Yard never find out. Now if you hada woman there
“As a matter of fact we have ”
“Yes, those dreadful policewomen in funny hats who bother people in parks. I
mean a woman at the head of things. Women know about crime.”
“They’re usually very successful criminals,” said Superintendent Battle.
“Keep their heads well. It’s amazing how they’ll brazen things out.”
Mr. Shaitana laughed gently.
“Poison is a woman’s weapon,” he said. “There must be many secret women
poisoners–never found out.”
“Of course there are,” said Mrs. Oliver happily, helping herself lavishly to a mousse of foie gras.

The firelight gleamed on the crystal stoppers. Always an artist in lighting, Mr. Shaitana had simulated the appearance of a merely firelit room. A small shaded lamp at his elbow gave him light to read by if he so desired. Discreet floodlighting gave the room a subdued glow. A slightly stronger light shone over the bridge table, from whence the monotonous
ejaculations continued.

“It’s impossible!” cried Mrs. Oliver. “Absolutely impossible. None of those
people can be criminals.”Superintendent Battle shook his head thoughtfully.”I wouldn’t be so sure of that, Mrs. Oliver. Murderers look and behave very  much like everybody else. Nice, quiet, well-behaved, reasonable folk very often.” “In that case, it’s Dr. Roberts,” said Mrs. Oliver firmly. “I felt instinctively that there was something wrong with that man as soon as I saw him. My instincts  never lie.”
“Didn’t get any extra change out of her,” commented Battle. “Put me in my place,
too. She’s the old-fashioned kind, full of consideration for others, but arrogant as
the devil! I can’t believe she did it, but you never know! She’s got plenty of
resolution.

“I should have kept him to the end,” said Mrs. Oliver. “In a book, I mean,”
she added apologetically.
“Real life’s a bit different,” said Battle.
“I know,” said Mrs. Oliver. “Badly constructed.”