The Mad Dog of the Duke's Estate

Kain raised his sword for a king consumed by madness, only to meet his end in the hands of a friend. But when he opened his eyes, he... was reborn as his friend's youngest grandson? No more being a loyal dog. Reborn as Karon, he'll throw away the leash, loyalty, and fate—and live exactly as he pleases.

His past life's sins are paid for: his life is his to wreak havoc as he see fit.

(Source: Kakao Entertainment, translated)