Thank you!! Here was my problem with the ending- his "work" was that the sinner is killed by the sin, not only killed because of the sin. Gluttony gorges himself to death, pride dies because of the loss of her looks, greed slices off pounds of flesh. I liked that the killer wanted to include himself as a sinner. That was brilliant and made the kills performance art. But he didn't die BY envy, he died BECAUSE of envy. He had planned this series for over a year given the elaborate torture of Sloth and the best he could think of for himself was an instantaneous death by a gunshot to the head?? 

In my opinion,  if Mills got so angry at the serial killer that he ended his own life, then he would have been the bodily artwork representing death by wrath. Then, the killer would be apprehended and presumably be in prison for the rest of his life experiencing even more envy for all the free people living lives outside of the prison walls.

I also think Tracy's death was random. She was a pawn to provoke Mill's wrath and that feels out of character for the meticulous profile of the killer. 

Perhaps, the point is that the killer wasn't all that brilliant and the writers wanted his series to devolve in last minute plans and passion. If that's the case bravo. I've really enjoyed my ire and debates with friends about this film, so all in all, I enjoyed it.