He probably never dreamed that she would ask for the head of John the Baptist. However, she did ask for it because her mother had instructed her to. I guess her conscience was pricked every time John told her it was wrong for her and Herod to be together. The king was sorry, but because of the oath he had given in front of all his guests, he commanded that John's head to be given to her.
We should never swear an oath to do anything. Sometimes it doesn't turn out the way we thought it would and we may be forced to do something we are ashamed of. In Matthew 5:34, Jesus said we are not to swear at all. He went on to say that our communication should only consist of yes or no. Anything apart from that is evil. Jesus was telling the people that it was very important for them to watch what they said.
How many times have we told God that if He did something for us, we would do a certain thing for Him. We shouldn't do this because we might not keep the promise all of the time. Be very careful what you say.