Marcello had told Folco quite plainly that he meant to be the master, and that he was of age, and should regulate his own life as he pleased, and he had expressed considerable disgust at the existence Folco had been leading in Paris and elsewhere; and Folco had always tried to laugh it off, calling Marcello prudish and hypersensitive in matters of morality, which he certainly was not.

Once he had attempted an appeal to Marcello's former affection, recalling his mother's love for them both, but a look had come into the young man's eyes just then which even Corbario did not care to face again, and the relations between the two had become more strained from that time on.
It might seem almost incredible that a man capable of the crimes Corbario had committed in cold blood, for a settled purpose, should show so little power of following the purpose to its accomplishment after clearing the way to it by a murder; but every one who has had to do with criminals is aware that after any great exertion of destructive energy they are peculiarly subject to a long reaction of weakness which very often leads to their own destruction.

If this were not a natural law, if criminals could exert continually the same energy and command the same superhuman cunning which momentarily helped them to perpetrate a crime, the world would be in danger of being possessed and ruled by them, instead of being mercifully, and perhaps too much, inclined to treat them as degenerates and madmen.

Their conduct after committing a murder, for instance, seems to depend much more on their nerves than on their intelligence, and the time almost invariably comes when their nerves break down.

It is upon the moment when this collapse of the will sets in that the really experienced detective counts, knowing that it may be hastened or retarded by circumstances quite beyond the murderer's control.