Why do you think Shaw's friend (also an officer) protests against the use of the whip for punishing the alleged deserter (Denzel Washington)? Common punishment for desertion (leaving the military camp) was to be flogged, or hit, but not necessarily with a whip. What about the whip is particularly bothersome?

The reason Shaw's friend protests against the use of whip on the deserter, Trip (Denzel Washington) was because he saw it inappropriate considering the fact that Trip was an ex-slave that had been whipped mercilessly countless number of times before.

The whip is particularly bothersome because it leaves the victim with tears and scars on the body.

Also, it was a form of punishment used for slaves.

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