That wasn't Leto's reason at all. He kept a standing army of women explicitly because according to him, women would be loyal to him as an individual while men would be loyal to the idea of the empire. That was counterproductive to his plans since he was literally trying to be the worst ruler for humanity in all of history.

But why are they violent? Latent (homo)sexual energy, apparently. I can't remember the exact quote, but that's ol frank for ya.

Edit: "The all-male army has a tendency toward homosexual activities" Something about the "breeding males" (older men) sending younger "non-breeding males" to war as a "screening function". So then you have all these young guys who have some options with what to do with their pent up energy when denied an "external enemy" - either be gay (apparently) or, since that is discouraged, they "sublimate" those energies raping and pillaging - hence being dangerous.