Question

how did sadao feel that the general was in the palm of his hand​

Answer 1

Dr. Sadao understood that the General was in his control, as the latter thought it was due to his own carelessness that the American man had escaped unhurt in spite of his promise to Dr. Sadao about killing the man.

Answer 2

It means that Dr. Sadao understood that the General was in his control, as the latter thought it was due to his own carelessness that the American man had escaped unhurt in spite of his promise to Dr. Sadao about killing the man.