The origin of bloodlust

I think the singular Japanese Elm represents his great grandpa’s first taste for humans. Being trapped in the trenches of war he probably had no other choice but to eat his comrades to survive, developing a taste for blood and flesh. But it became hereditary supposedly right? Maybe cannibalism is hereditary like in the movie “Raw”, or maybe they’re more like vampires since that can also be hereditary. They don’t inherently have a taste for humans though, it gets triggered after their first time. I don’t know how the grandpa and dad got triggered, but Kei definitely was triggered when he tasted his own brother’s blood. And now that Sai has had a taste, maybe he too will become the same. I wonder though, is the title of “Double Suicide” referring to the fact that two people have committed it in the story, or will Kei and Sai eventually fall deep into their bloody desires and, unintentionally or not, commit double suicide?
This was a really interesting story. Although it was a bit hard to read personally because of the themes of self-harm, it was still really intriguing.