While Kamiki certainly deserves to die, I don't killing a criminal should be the way to go.
Especially since that'd be jeopardising the efforts of Akane and co. trying to gather evidence rn.
Then again, it's just as Aqua said, his father is wicked, acting in a way to be hardly/almost impossible to prosecute. Even more so with the only witnesses still alive being Nino who's far from mentally stable herself.
It's really bad decisions only:
Leaving him to his own devices, he's a fatal danger, killing him would have serious implications and getting him locked up (the best solution) is really difficult/impossible...