But if he is such a powerhouse why not sent him out to do some suicide like missions? Varis himself said that Zenos was not fit to sit on the throne yet he gave him such a position. And since he was able to take out all the samurai in the rebellion in Doma in the past, he was already a strong fighter.

So if you want to conquer Eorzea or other continents why not just sent him and some other fight loving soldiers there to cause some unrest. If the people are too weak, Zenos might have claimed some land and if not you would have only lost a "monster" anyway. Yet he let him lead Ala Mhigo and Doma with a very bloody hand which will surely cause destabilization and unrest in these countries and maybe even among their own soldiers.