Corrupt the Cutie: He agrees to feed people to Audrey II to keep it alive and win Audrey's affections. However, after he feeds Audrey and realizes what Audrey II is planning, he decides to try destroying it. He fails in the plays, but succeeds in the theatrical cut of the movie.
Defeat by Modesty: In both versions of the film, Audrey II thwarts Seymour's attempt to ax it by pantsing him, prompting Seymour to drop the ax and duck behind a counter.
Karma Houdini: Seymour killed two people (he let Orin die despite making it clear that he could have helped him and he backed Mushnik into the plant) and considered going even further (he signed the contracts in "The Meek Shall Inherit", implicitly agreeing to kill others in the future. This is made clearer in the play where the song is extended, but the actions remain the same in the film.) but in the theatrical cut, he gets away with absolutely no punishment whatsoever-Audrey likely never even learns that he is a murderer.
It can be argued that Seymour finally standing up to and destroying the plant thus preventing the destruction of the human race pretty much makes up for the world being less one sadistic dentist (which WASN'T his fault, despite intentions to the contrary) and one greedy, albeit somewhat kind, shopkeep (which may not have been entirely on purpose whatever the plant said). Both deaths are made a lot more ambiguous in the film than the play, and can be interpreted as due to Seymour's actions or not.
The Director's Cut makes it pretty clear that he didn't get away with anything, even if you don't count his death. Orin and Mushnik died only because Seymour doesn't prevent their deaths, and they both would have harmed him (Orin with dental sadism, Mushnik through blackmail) even if he had saved them. Audrey, on the other hand, he consciously fed to the plant, even if it was at her own selfless request. If he hadn't done it, Audrey II might not have been strong enough to tear down the building and eat Seymour whole.
In a deleted scene, Seymour during the "The Meek Shall Inherit" scene does grab an axe to destroy Audrey II, but the thought of losing Audrey stays his hand.
Laser-Guided Karma: In the original 1960 film, Seymour is one of the buds when Audrey Jr. blooms, along with the other people he fed to the plant.
My God, What Have I Done?: The film gradually leads Seymour to this. He's horrified that he chopped up Scrivello and he led his boss into Audrey II's jaws. Eventually, he realizes that he's gone too far and doesn't want to continue with what he's been doing.
Nice Guy: Once you get past the "grows a giant man-eating plant that needs blood to stay alive" thing.
Senseless Sacrifice: In all versions save the 1986 film, Seymour tries to kill the plant by letting himself get eaten (or charging into the plants maw) with an axe to hack from the inside. It doesn't work.
All Girls Want Bad Boys: Averted. Even though she dislikes being in a relationship with Orin, she just can't break it off, because she fears his reaction to the breakup. She also states she doesn't deserve a good relationship.
Jerk With A Heart Of Jerk: He still treats Seymour like crap, even after Seymour is solely responsible for saving the flower shop, and tries to get rid of Seymour so he can feed Audrey II himself. He does, but not in the way he thought. At least in the movie — in the play, Mushnik has good reason to believe Seymour killed Orin and is simply doing his duty as a citizen by telling the police what he knows, and he gives Seymour the opportunity to come with him and defend himself, in case it really isn't what it looks like. Seymour tricks him into getting too close to the plant, purely to save his own ass.
Enfante Terrible/Creepy Child: "When I was young and just a bad little kid, my Momma noticed funny things I did, like shooting puppies with a BB gun, I'd poison guppies and when I was done, I'd find a pussy cat and bash it's head, that's when my Momma said..."
Pet the Dog: When he first meets Seymour, he bullies him a bit, however in two instances he was nice to him in his own depraved way. Before knowing who he is, he offers him some Nitrous Oxide, something he doesn't even give his patients. When he does know him he acts starstruck and is genuinely amazed that Seymour was able to have a plant as big as Audrey 2.
What the Hell, Hero?: He has this attitude towards Seymour in the musical. Rather than help him get the mask, Seymour decides to let Orin suffocate from the gas. This line of dialogue certainly warrants this trope:
Orin: Hey, Seymour, give me a hand, will ya?
Orin: ...Well? He says, "well"?
Crystal, Ronette, and Chiffon
Played By: No one in 1960 / Tichina Arnold, Michelle Weeks, and Tisha Campbell-Martin
Greek Chorus: An interesting example, going back and forth between standing outside the action and commenting on it to the audience, and interacting with the other cast members using no special out-of-character knowledge. You can keep track by the costumes; when they're characters the girls wear worn-down clothing appropriate to residents of Skid Row, and when they're a Greek Chorus they've changed into sparkly dresses.
Expy: Of Wilbur Force, a masochistic dental patient Jack Nicholson portrayed in the 1960 movie.
Freudian Excuse: His masochism may or may not have been cuased by his child dentist's habit of giving him candy bars after painful procedures.
Jerk with a Heart of Gold: While he is a Masochist, he understands not everyone is as enthusiastic to pain as he is. And since he opted for the most painful procedure on himself, he honestly considers the poor kid with the wired jaw to be considered lucky.