I haven't seen Ichi the Killer in awhile, so the roof scene is still kind of blurry to me (in remembering the consecutive order of the scenes particularly); but IIRC, Kakihara actually killed Ichi, then stuck the pins in his ears so he wouldn't have to hear how pitiful Ichi's crying was. Kakihara was so dissapointed with Ichi that he then "imagined" being killed by Ichi; committing suicide later by throwing himself off the roof (which explains why he has no scar on his forehead).
The boy at the end I thought was supposed to be the son of the gunman. You see the boy feeding a crow in one scene of the movie (crows being bringers of death in mythology) so you're lead to believe he's harboring a need to kill someone. At the end, when Jiji is hanging from the tree, what (I assume) is the same crow flies by or perches on the tree (I forget exactly). Also, Jiji being hanged similar to a gallows setting sort of makes it seem like a "judgment" or "justice" killing; so again, you're left to assume it is the gunman's son out for revenge.
I don't see how anyone can find this movie "tame" considering the violence, rape scenes, etc. Miike gets more sick, gross, disturbing, etc. with each of his movies to me (which is one of the attractions to watching them I guess) so I have to wonder what the hell you are watching (Faces of Death?) if you think anything by him is "tame" LOL
If you really feel left out after seeing Ichi the Killer, then Dead or Alive has some more "sick" or gross-you-out kind of stuff. I'd also recommend seeing Audition if you're looking for a more "serious" Miike film.