I guess I get Ken's motivation in wanting his "family" back. But he already wanted to cut Moe out of it, and all of this requires ignoring everything his mother has done, that all of them are presently aware of. Whatever he was trying to put together, it wouldn't be happy, and he has serious tunnel vision in thinking any of this would be good for any of them.
It just kinda comes down to petty selfishness on his part. I don't begrudge him the desire for some semblance of stability and normality, but this family was never going to give it to him. He really should have just left with the detective and never looked back, and now that he's resorted to abduction and pushing this false equivalency choice upon his "father" (jury's out on whether Gotou even is, at this point), I can't say he's one of the few remaining examples of "good person" in this story. (That title is reserved for the two investigators, in my mind.)
I think I only feel pity for him, at this point, in the face of his refusal to learn that he has to make choices, and not simply try to force them onto others.