Justice.
A word, for many. While it may be endlessly debated and poured over by philosophers, for the most part, a word is all it is. Just a vague, undefined quality trotted out in speeches to make them sound authentic.
A word.
That is not what it should be.
He doesn't pretend to know everything about his chosen path. He knows that justice is not a way of life, nor some elemental force. It does not hold any power of its own. Rather, it is an ideal.
However, it does seem to possess a peculiar karma of its own. Specifically, those who harm others receive their own due in time. Their very nature is their undoing. Killers die by another's hand. Thieves lose all they have gathered. Torturers who delight in their trade find themselves, sooner or later, at the mercy of another of their ilk. It is a cycle, a constant never-ending circle.
He knows this well. It has served as his only consolation, when she died. When he stood over her corpse and did his best not to cry, the only thing that kept him from joining her was the knowledge that eventually, her worthless husband would get his richly-deserved reward.
He waited and waited, but despite numerous attempts and his own boorish lifestyle, the man-shaped excrement refused to receive his just deserts, swaggering around as if he had the right to even exist in this world.
So when the world refused to act, he did.
Only after the blood had dried and the corpse had been discovered did he realize the fundamental truth.
He was justice.
Justice is balance. Justice is fairness. In a world that is not fair and has no balance, there must be one who works to create them.
He does not like to fight, but he knows that fighting is necessary. So when he tells Sajin that the path he takes is that of justice, he means it. Why would he take a path that is inimical to who he is and what he upholds?
He knows that no one believes him when he says that his is the path of least bloodshed, but it is true, regardless. Soul Society is stagnant and corrupt, condemning hundreds of innocents to death each year in the Rukongai districts. Children starve and people are murdered while the 'noble' captains of the Seireitei posture and prance.
It is unacceptable. The misery and death in Soul Society is enough to eclipse all the deadliest wars in the history of human memory, and will continue to eclipse them. Wars end, after a fashion, but government can endure for far longer, supported by the shoulders of good, but misguided men and women.
He is blind, but he is the only one who truly sees. Aizen-sama is the only one besides him who can acknowledge this, who knows that, even in the struggle with Soul Society and the countless souls who will die in it, the body count and the bloodshed would still be lower than if Yamamoto's precious government is allowed to continue.
He doesn't really know what Aizen-sama wants. He's not so naïve to believe his leader would topple the Seireitei to alleviate human suffering. Though the Fifth's former captain has stated he wishes to become God, he doubts that is all. Aizen has never been straightforward, and never will be. Nevertheless, if following him will lead to peace with the least amount of bloodshed, he is happy to do so, even if it means the slandering of his name and the ruination of his character to all generations who come.
Kaname Tousen is a selfless man. He is willing to sacrifice all that is his, and even all that is not, to achieve his goal.
It has never occurred to him that he has fallen into the oldest sin. Arrogance. He believes that he is justice, that he is the only one who sees what must be done.
In a world full of the blind, he is perhaps the blindest of all.