The emergence of Akujin surviving the attack was unknown to anyone as they could barely make him out but Kenzo knew in his guts that if anyone was going to survive something like that.
It had to be Akujin, there was no other person but this would derail his initial objective if Akujin survived, it could be to his advantage.
Akujin was the last person to speak to Gerard but that could also enforce the idea that Kenzo had something to do with it.
Kenzo didn't take his life after all, Kenzo was aware of this possibility but they both went back so he doubted that Akujin would attempt to claim his life.
Kenzo had a plan and his revulsion for the Don wasn't a secret but he knew he had to move smart or it could blow up in his face.
But in actuality, he wanted to take over the Yakuza because on the off chance that he couldn't stop the apocalyptic event that was about to take place.