Easy. Don’t just have them rant about what they are doing. Have them tell their audience why they are doing it as part of a persuasive argument. Either they are rallying their allies/underlings or trying to convince nonbelievers that their motives and actions are just.
Perhaps the best method is to avoid the monologue structure entirely. Break it up with actions and feedback from other characters as part of a conversation. Here’s how I had one of my antagonists explain her plan/actions in a recent work:
The Path to Enlightenment
“You have no need to feel guilt for your skepticism, Danna. Your open mind is right to question that which tempts you. You seek true enlightenment, and so you must discover the candor within your own heart. Did you know most of those in the Light bear none of my mana at all. They are called by their hearts alone. Now, tell me, would so many offer themselves to me of their own volition if my cause is not just?”
When Danna didn’t answer, Wu’laujan went on.
“Do you know why the Army of Light exists? It is because Darkness threatens the souls of the innocent. We are the sentinels who guard them against temptation, against sin, against damnation. We are the path to salvation; not a force of oppression from beyond the realm of humanity. We are a militia formed from the ranks of mankind, ever vigilant, ever prepared to hold the light that will guide the lost from the darkness of desolation. We will always be ready, because we will always be here.”
“But you’re trying to build a false utopia!”
“Oh? Are those your words or another’s?”
Danna swallowed a denial, knowing she was caught, but undeterred. “You say you want to help people, but my mother said that millions of people would have died in your last crusade. That’s why she had to stop you. If you’re trying to protect them, why would you do something to kill them?”
“Oh, my dear child, you do not understand. Mortal life is sacred and deserves protection, but it pales in comparison to the fate of the soul. The sanctity of the spirit must be preserved at all costs. In my vision, wickedness will be purged from the Earth by granting those tainted with demons a glimpse of purity. Upon embracing the Light, upon embracing goodness and wisdom, their souls will be cleansed. Only wretches of such vile heart who could never find a way out of evilness and darkness will be expunged from the world.
“Do you think it was an accident that I chose China to build my army? No, it was my design. China was my child. When I arrived, the people there were lost in nightmare, consumed by worship of meaningless gods. They had lost their way, but I gave them the discipline and technique they would need to fight the blackness that now threatens us all. They learned how to channel their spirits into a martial force.”
“You mean you started martial arts? But they’re so old.”
“More than that. It was not my time to fall, but I saw the Darkness coming. I formed a pact with my spirit council. They sacrificed their lives so that I, the strongest among us, could breach the barrier between our worlds. And so, I fell to Earth several thousand years ago. I fostered the culture of the Eastern world into a realm that would embrace the mission of the Light. I ignited their jiāodiǎn and gave them means to harness their spirit. I bred them into a people of focused mind with strict adherence to discipline, order, and code. The perfect soldiers to inherit the mana of Heaven. A unified force of single cogitation.
“Alas, my vision was halted by the yáng guǐzi. For him there was no forgiveness, but for your mother there is still hope. I call upon you to help her see the truth of my vision. China is lost to me for now, but here in the West, I find the people emboldened with a potent creativity and freedom of spirit that can be cultivated into a great power. It requires greater effort to unify them, but they may rise to become an inspired and diverse configuration of driven entity in time. Time we no longer possess. The Darkness has come.”
Wu’laujan sat for several moments, staring into space, and Danna didn’t know how to respond. The archon was obviously dedicated to her mission of helping the world, which didn’t seem unfounded to Danna anymore.