Sylas had actually wanted to "keep" Thessa. But the method with which he "killed" the Duchess and the Prince required a thorough, complete, and perfect understanding of their bodies and bloodline. That wasn't something he had had the time or stamina to do with Thessa. So…
He directly killed her.
However, the irony of that was that Sylas had killed her precisely because he didn't understand how her bloodline worked, so it wasn't exactly impossible for her to have pulled the wool over his eyes in some way, especially since he had been more worried about the coming challenge from Kairos and the Monkey King.
It didn't take long for Sylas to realize why it was that Thessa might keep the Duchess alive.
The idea of the Duchess hating him was very much a good chance, but even if that didn't work out… how would this land at Nosphaleen's doorstep?
Not long after he ultimately rejected her in favor of Cassarae, he not only took in another woman, but she was even pregnant with his child?
