On the Spot by Edgar Wallace

JIMMIE McGRATH found himself unlocking the door of his room without realizing how he came to be there. He was moving mechanically, thinking not at all. The world had become a monstrous unreality; the living people who thronged the streets and occupied the wild tangle of motor traffic, that moved and stopped obediently at the green and the red traffic lights, were of another kind to himself. They had homes and relatives and interests. Thousands of them were dressing for dinner at this moment; millions were relaxing in the security of their apartments and houses, having no worries beyond the domestic problems which beset every household.

He belonged to a people apart. They were human beings; he was something entirely different, a potential murderer, the cold--blooded assassin of a man who had trusted him, and probably had never trusted another. Well, he must get over that. Jimmie showed his teeth in a painful grin.