Interlude & Soujirou
When Soujiro meets Shishio, kills his family, and leaves his home, he was only eight years old.
From that time on, he chose to follow Shishio. There he enters the world where only the strongest survive and any others are killed. Nothing of this period is shown in the manga.
However, it must be noted that in the ten years he spent with Shishio (prior to the Kyoto Arc’s beginning), Soujirou came to terms with the life of an assassin and a killer. One cannot imagine what horrors he may have faced under Shishio’s command, and what experiences he had that could have led to what he became, 10 years after.
Thus has the boy, Seta Soujirou, come into his own under my tutelage. Let’s see if you can defeat him, Battousai. Let’s just see what you can do. [Shishio, ch128]