John did not know Herodias. He was commenting on Herod Antipas' visit to his half-brother Herod Phillip where Antipas fell in love with Phillip's wife and went through a few intrigues to get her to move into his area. Antipas intended to secretly divorce his current wife, but she got news of his plans and pretending she knew nothing, got Antipas to send her to visit her father.
While this may sound like a tempest in a teapot to us, it seems it was a big to-do at the time, because people started raising armies.
Meanwhile, John-the-Baptist was popular in his own right and Antipas thought he, too, might be able to raise an army, so he imprisoned John as he had enough to deal with between Herod Phillip and his former wife's father.
At this point, Antipas may or may not have been satisfied with just the imprisonment of John, but the account in the Bible suggests that Herodias was the force behind John's execution.