Soujiro was an unhappy child, severely abused and forced to work by his cruel relatives. But this is why he seems to be smiling all the time. He discovered that if he smiled instead of cried when they beat him, they would soon get tired and stop. So, in time, he started to smile whenever something bad happened.
Soujiro encountered Shishio killing policemen in the yard. Actually it was the smile that saved him from being killed himself. He hid Shishio in the barn and Shishio taught him the 'the strong shall live and the weak shall die' philosophy, gave him a sword, and encouraged him to defend for himself. Soujiro was going to return it when the family found out that he was hiding a criminal in the house and was going to kill him. Only at the age of eight, Soujiro had no other choice but to kill them first, and after that, followed Shishio.