Not to mention he kills the soldier that reports back to him after we liberate doma doesn't he?
I got the Chronicles of LIght book, and there's a short story about Zenos.
In it, he was a prodigy genius as a child (of course). Mother died while young and Varis rarely had time for him and he basically saw his servants as automatons.
Then when he was 14, he got a new combat instructore. A master swordsman from southern othard. He kicks Zenos' butt every day and at first he can't even follow his movements. Then little by little Zenos starts to learn and improve, until he's able to fend for himself more. He starts studying the swordsman's style and eventually learns it by examining him, even if it means allowing himself to get hit sometimes in training to get a better look.
He's eventually able to replicate the style, even the aether use, by embedding a crystal into one of his palms. Said to be dangerous but Zenos is able to use that to force his body to release aether. The conscripted trainer flips out that Zenos is learning his special sword style and attempts to kill him but Zenos defeats him and kills him. It also plants the idea i Zenos' head that the inability of garleans to use aether naturally is a weakness that needs to be dealt with.
Ultimately nothing all that informative, doesn't really add any more layers to his character we didn't know of before or couldn't have inferred from the game other than I guess the crystal in flesh thing. Though I do wonder when Zenos got his resonant abilities. All of the facilities related to it seem to be in gyr abania and I'm not sure Gaius would have allowed those experiments so it makes me wonder if maybe it was between 2.0 and 4.0. Not sure if it was before or after he crushes the first Doma rebellion.