Mark 6:16-20

Mark 6:16 (KJB)
But when Herod heard thereof, he said, It is John, whom I beheaded: he is risen from the dead.

Herod was insistent that it was John the Baptist. This is because his conscience bothered him because of his acquiescence to the request by Herodias but then again, in front of all his guests, what recourse did he have. If he refused to do it, he would have lost face in front of all his guests and especially Herodias.

Mark 6:17 (KJB)
For Herod himself had sent forth and laid hold upon John, and bound him in prison for Herodias' sake, his brother Philip's wife: for he had married her.

The reason that Herod had feared that John the Baptist was risen because of his marital status. He had divorced his wife, the daughter of Arabian King Aretas IV of Nabataea. (2 Cor 11:32 KJV) In Damascus the governor under Aretas the king kept the city of the Damascenes with a garrison, desirous to apprehend me: His rule was in the southeast portion of the Dead Sea. He had John the Baptist imprisoned because he told Herod that he should not marry his brotherís wife.

Mark 6:18 (KJB)
For John had said unto Herod, It is not lawful for thee to have thy brother's wife.

John knew the law and told Herod that it was unlawful for him to have his brotherís wife. Herod then proceeded to marry Herodias who was the wife of his half-brother Philip and then sent his divorced wife back to Aretas. According to Leviticus 18:6-18, Herod was forbidden from marrying Herodias because they were family through Philip. They also violated two other laws by doing what they did. (Lev 18:16 KJV) Thou shalt not uncover the nakedness of thy brother's wife: it is thy brother's nakedness. This was basically a marriage of incest. (Lev 20:21 KJV) And if a man shall take his brother's wife, it is an unclean thing: he hath uncovered his brother's nakedness; they shall be childless.

Mark 6:19 (KJB)
Therefore Herodias had a quarrel against him, and would have killed him; but she could not:

Johnís message had infuriated Herodias and if she had the authority, she would have had him killed herself. The Herod line were filled with violent, uncompassionate people and Herodias would have had no problem in killing John. They even killed their own family members. After all, she did not even take a second to think about what her daughter should ask for from the king. She instantly told her to ask for the head of John the Baptist on a charger.

Mark 6:20 (KJB)
For Herod feared John, knowing that he was a just man and an holy, and observed him; and when he heard him, he did many things, and heard him gladly.

Herod had feared John because no doubt John was telling Herod about the fact that he would someday stand for judgment because of the things which he was doing in his kingship. He knew deep down that John was guilty of nothing more than bringing the message of warning to Herod. John was becoming the conscience of Herod concerning his illegal marriage to Herodias. Herod did not want to put John to death because he knew that the crowds would castigate him since all those who went out to the river Jordan and heard John speak, had regarded him as a prophet and since he was regarded as a prophet, then Herod would be accused of killing a prophet of God and he did not want that on his record. It could also be that Herod was protecting John from Herodias who eventually may have decided to pay someone to kill John knowing the evilness of this woman. Herod had listened to John and as a result he had done many things. Now it is not known exactly what those things are but maybe they were acts of kindness and Herod was beginning to soften a little bit in his rule. This may have been spotted by Herodias and that is why she wanted John out of the way so the influence for good would be gone. It seemed the voice of truth which he heard so gladly was also the voice which was causing him much emotional torture.