The official Stormblood site has this to say about Zenos:

"Leader of the XIIth Legion and crown prince of the Garlean Empire, Zenos is possessed of but a single passion: to stand on the front lines, blade in hand, and dance with death. He regards friend and foe alike with contempt, and tales of his brutality against both have spread far and wide. While crushing the rebellion of yesteryear in Doma, he took a liking to Far Eastern katana, which have since become his weapons of choice."

Zenos was also shown to be rather intelligent, or at least possessed of a thorough spy network, as he correctly identifies Shinryu by name presumably mere hours after its appearance. That he enjoys standing on the front line suggests he is also a formidable opponent, and the trailer strongly hints that he indeed is - Zenos is seen fighting Lyse, who is saved only by a timely intervention by Y'shtola, as well as Yugiri, capable fighters all.

That said I don't believe Zenos is malevolent towards his people - he's apathetic, if anything, towards others in general. He fights not for conquest or survival, but for sport. I do hope there is more to him than that, but based on his actions in the past and the response doing nothing elicited from his adjutant, I think he's established himself as a credible threat.