He tells as that we must "learn to know every man's thumbscrew." I suppose you know that a thumbscrew was an instrument of torture used in old times to force confessions from criminals.

This advice means nothing less than that we should learn how to be be able to hurt other men's feelings, or to flatter other men's weaknesses.

"First guess every man's ruling passion, appeal to it by a word, set it in motion by temptation, and you will infallibly give checkmate to his freedom of will." The term "give checkmate" is taken from the game of chess, and must here be understood as meaning to overcome, to conquer.

A kindred piece of advice is "keep a store of sarcasms, and know how to use them." Indeed he tells us that this is the point of greatest tact in human intercourse.
"Struck by the slightest word of this kind, many fall away from the closest intimacy with superiors or inferiors, which intimacy could not be in the slightest shaken by a whole conspiracy of popular insinuation or private malevolence." In other words, you can more quickly destroy a man's friendship by one word of sarcasm than by any amount of intrigue.

Does not this read very much like sheer wickedness?
Certainly it does; but the author would have told you that you must fight the wicked with their own weapons.