Why doesn't Naraku "win"

[Wio]

So far in the story, Naraku has accomplished most of his more significant goals. My question is "Why doesn't he just WIN already?"
Perhaps it is just me, but it seems that there were many times where Naraku could have killed Inuyasha and friends but gave them mercy for whatever purpose. However, I no longer see any purpose to keep them alive. Hasn't he tainted the Shikkon enough yet?

[TheAsterisk!]

He likely enjoys playing with their minds, as do all evil masterminds.

[Animeswordmaster]

He knows that he is powerful enough to kill them (or at least thinks he is) and only wants to toy with their minds.

[Higurashi Kagome]

He has the entire jewel and you know what he said his whole purpose was? To feed the shikon no tama with the fear, hatred and darkness of the inu-gumi.... -.-;;; The lame *** has the power to kill them, the ability and now the utmost opportunity and all he wants to do is taint the jewel more with his persuers...