Sylas wasn't nearly as mad as Cassarae thought he was. That wasn't because he was so sure that she was lying either. After all, he hadn't been by Cassarae's side in 10 years. In that time, she had finished off her last year of high school and gone off to campus. Who knew what happened or how she could have changed in that time? He wasn't naive enough to be sure she had been some holy saintess the entire time.
However, if Sylas was going to make his decisions based on that, he would have never put his foot down in the first place. These were simply things that he just didn't give a damn about.
There was a primal desire in him to be the first and the only, but… there was an even stronger primal desire in him to be the best and the last.
Cassarae also represented something else to him, something that he wasn't quite able to put into words. Maybe if he was able to do that, his Madness Comprehension would be Gold instead of Silver.