The main idea in "The Use of Force" by William Carlos Williams is that the use of force itself, no matter how morally good the purpose, corrupts the user. The story starts out portraying the narrator as an admirable person, a hard-working country doctor concerned about his patients. Due to the diphtheria outbreak at the local school, he is very concerned about the young girl, Mathilda Olson. He describes the child as attractive but somewhat hostile to him or suspicious of him, looking at him with "cold, steady eyes."

The first use of force in the story is actually by the girl, who lashes out at the doctor as he tries to get close enough to take a throat culture:

As I moved my chair a little nearer suddenly with one catlike movement both her hands clawed instinctively for my eyes and she almost reached them too. In fact she knocked my glasses flying and they fell, though unbroken, several feet away from me on the kitchen floor.

Once force is used, the story seems to indicate, it can only escalate. The narrator thinks: "Then the battle began. I had to do it. I had to have a throat culture for her own protection." Even though the doctor's aim is to help the patient, the need to get a throat culture has become framed as a battle, and the struggle would continue to escalate until a clear winner was established. The doctor loses his temper as the child becomes increasingly hysterical, and eventually manages to get the throat culture by forcing the child's mouth open with a metal spoon. 

The concluding description uses almost horrifically brutal imagery:

The doctor sees that it would be best to stop and come back later, but he is beyond reason, and in his fury he asks for a makeshift tongue depressor that she cannot destroy: a spoon. In spite of Mathilda’s bleeding mouth and hysterical shrieks, he persists ...

What makes this passage particularly disturbing is the way the necessity of using force for a good purpose has unleashed an almost primitive violence in the doctor and the girl.