Se7en was a celebration of human suffering. It reveled in the pain of the victims, examining it in excruciating detail. Not that Se7en is unique in this, but this movie did it for no morally-justifiable reason. What I mean by that is that, in the end, the bad-guy won - there was no moral message to the movie, no lesson to be learned, no coming out of the movie a better person than you went in.
This can be said of any gore/slasher movie, but Se7en was not a gore/slasher movie. Gore/slashers are not meant to be taken seriously - they're simply dark entertainment. Texas Chainsaw massacre etc. are a genre unto themselves and you go in suspending your mores.
Se7ev dragged your morals in with you, set everything up in a realistic world for you to judge good and evil, and then simply showed suffering. It is difficult for a person to comew out of the film without being polluted by it - and not in a good way.
It was the realization that I no longer had the need to be stimulated this way that caused me to forego psycho killer stuff thereafter. I've seen enough of the world now that I don't need to get my pain and suffering artificially. It doesn't mean I don't happily watch movies of all types - eg. I absolutely loved Kill Bill and Sin City (they were cartoons, not drama). But Se7en was bereft of all the elements that put suffering in context.
I guess, to use a (bizarre) pr0n analogy, I have lost interest in the glaring, faceless sexual-zombie extreme-closeup explicitness of Hustler, and have come to prefer the more subtle and alluring, teasing eroticism of Playboy.