As of the end of Shadowbringers(patch 5.55) storyline you havent been told much about the motivations of our antagonists. When he introduced himself, Fandanial told everyone present(you included) that he wanted "you all to die. And i want to die too". He doesnt give his reasoning. As for Zenos, he killed his father because his father was in the way of his greatest wish("any and all who interfere with my hunt... will not do so twice!"). His grand fight with you at the Royal Menagerie was the highlight of his life, and he will do whatever it takes to get a rematch. You also saw several cutscenes hinting that more was going on with both of them, but whatever that actually is you havent been shown yet.

And yeah, you should have your answers over the next few levels of MSQ. Enjoy the ride!