Your hands shake. Are you really about to do this? You take a deep breath. You have to do this, you can't chance that he wakes up good. You're doing this for Duchess, Moki, the Sisters and for yourself.
You raise the stake up high, the shadow of the stick covers his face. With all your might, you plunge it down on his neck. It easily penetrates through his throat and sticks into the ground. Blood gushes down the stick and pools on the ground. Duke wakes up suddenly, eyes wide open. He looks at you, then the stick. He looks terrified.
He gurgles out blood. His legs are tense and twitching. He looks back at you. His eyes soften and a bloody faint smile spreads across his face. Blood pours out of his mouth. He gurgles out his last words, "and I... thought I was the monster." The smile fades and he closes his eyes. He twitches then slowly goes limp. He relieves himself as he dies. One of his back paws twitches after his death.
You stumble back. You just killed him. You heave but nothing comes out, you cry. He was right, you are a monster but not like Duke. You are the monster that kills to protect your friends, he is the kind of monster that kills his friends.
You get up and sigh. You did what you did and you don't regret it. You don't look at Duke as you walk away. You take a deep breath. This is the way of the world. The strong get what they want and the weak suffer the consequences. A new resolve has formed in you. Your quest for a Thunder Stone is a good one. You will use the power you gain from it to protect the ones you love and bring vengeance on the wicked. This was just a taste of what you will become. You head back to the fork in the path.