Re: Staying soft.
I cannot site anything other than word of mouth for the actual kills, but it is not a jump in logic. The numbers are purely what I remember from what I have read/heard, I could be wrong about the number. O Sensei is reported to have been in numerous duels. These are refrenced in most of the books out there. At that particular time duels were still to the death as most samurai traditions were alive and well in the martial arts. He won every duel. Had he lost, we would not have had an O Sensei, just a talented youth who bit off more than he could chew.
From personal accounts of his students, he had a world class temper until he had white hair. We tend to think of O Sensei in terms of his last decade of life, not the vibrant youth who fought in the WWs, who did all the work that led to the O Sensei we all revere.