Many homicidal lunatics are very quiet, unassuming people. Delightful fellows. - Agatha Christie

I'm used to that. It often seems to me that's all detective work is — wiping out your false starts and beginning again.

Motives for murder are sometimes very trivial, Madame.” “What are the most usual motives, Monsieur Poirot?” “Most frequent — money. That is to say, gain in its various ramifications. Then there is revenge — and love, and fear, and pure hate, and beneficence — ” “Monsieur Poirot!” “Oh, yes, Madame. I have known of — shall we say A? — being removed by B solely in order to benefit C. Political murders often come under the same heading. Someone is considered to be harmful to civilization and is removed on that account. Such people forget that life and death are the affair of the good God.

Every murderer is probably somebody's old friend.