How does Robert de Kere change as a result of his many trips in the time machine in the novel, Timeline?

As a result of taking too many trips in the machine, Rob has grown increasingly violent. He has a seam on his face that causes him near constant pain. Rob got into a bar fight a year before this novel begins and decided to run away by going through the machine one more time. Rob has become a part of Sir Oliver’s army, acting as an advisor to Sir Oliver. When the professor, Marek, Kate, and Chris come to Castelgard, Rob recognizes them as modern people and makes it his mission to kill them. Instead, Rob, as de Kere, is killed by Chris.