And when he would have put him to death, he feared the multitude (cf. Luke 20:6). Mark has, "And Herodias set herself against him, and would have put him to death; and she could not; for Herod feared John."
The more detailed account in Mark is doubtless the more exact. Perhaps the facts of the case were that, in the first heat of his resentment, Herod wished to kill John, but feared the anger of the people, and that afterwards, when he him in his power and Herodias still urged his death, Herod had himself learned to respect him.