So he needed men that don't fear death and play it dirty on the battlefield, anything to kill and win. Don't let your dignities get in the way, survival is key. The samurai where greatly feared for this reason, skillful, no fear of death, masters in tactics, heavily equipped and not forget being able to cut one in half with a katana. The horror of cut limbs and bodies probably roamed as stories around for centuries. Fear is a strong tool. They made use of this human natural instinct. They still use it today to keep everyone in check with the government these days. It's called smart power, very effective.

https://en.m.wikipedia.org/wiki/Smart_power