What do you understand by the term 'fatal instinct' in the story "The Case For The Defence" by Graham Greene?

The phrase "fatal instinct" is found in the third
paragraph of the short story "The Case for the
Defence".

The story opens as the first-person narrator
tells about a murder trial which they attended. The man accused of the murder of an old
woman was "all but found with the body."

The phrase "fatal
instinct" is used to describe the fact that Adams (the murderer) is caught with the
weapon he used to murder the old woman. A neighbor (Mrs. Salmon), thinking she heard a
noise at her own gate, sees Adams leaving the home of the woman he murdered. Adams feels
a "fatal instinct" as he looks up to see Mrs. Salmon watching
him.

Adams recognizes the fatal (deadly) instinct
(intuition) that he has just made a mistake. Being seen by Mrs. Salmon puts him at the
scene at the time of death.

Typically, fatal instinct is
what keeps animals alive. The knowledge of something existing in such a way that could
potentially end ones life. This is the same for Adams. He knows that Mrs. Salmon could
be a witness which could out "a nail in his coffin."