You´re completely right with your thoughts. After the fifth sin the causal chain gets illogical and inconsistent.
Here´s the list:
1) An obese man gets punished (killed) by "John Doe” for GLUTTONY
2) A wealthy man gets punished (killed) for his GREED
3) A drug dealer gets punished (killed) for SLOTH
4) A prostitute gets punished (killed) for LUST
5) A model gets punished (killed) for PRIDE
(but now it gets inconsistent:)
6) The sixth murder - if this line would stay consistent - should be someone getting punished for ENVY. but "John Doe” murders Mills´ girlfriend, which makes no sense in the causal chain… she is not the victim because of her envious behaviour, she´s just a plot-tool so envy does not get punished at this point at all.
7) The seventh sin would be WRATH, now Mills kills John Doe, which actually now takes care of point six, ENVY.
- What we have here is an innocent for kill number 6 who is not related to ENVY herself.
- Kill number 7 should be WRATH, but clears up number 6, ENVY.
- and sinner number seven, WRATH (personified by Mills), is not killed at all, his punishment is the loss of his wife and child.
Great you realized that, most people don´t.