Tengu aren't exactly considered "citizens" or really "people" on the planet where Kane grew up. Maybe that's why his island was raided and he was captured as a trophy by monster hunters when he was just 4. What they didn't know was they might have done that world a favor. By the time he was 20 Kane had taken over a kingdom by regicide and established world peace in under a year.
Chapter two, during his travels he was bound to an incredibly powerful weapon, one that could kill gods and it was sentient...and hungry for his soul. He managed to keep it at bay and his mind his own for a few years but one day it was just too much and it took control of him and made him go on an interdimensional killing spree, for a few years. Along the way he killed too many for him to even remember before he found himself on a little world where he met a few people who would become VERY important to him...