These views are my own and are affiliated with no one involved in the production of Heathers.
 
"Greetings and Salutations; are you a Heather?"
Is J.D. really a Heather in disguise?  Not exactly; but there are similarities.  First off, a sort of "sympathy for the Devil," is conjured up in both J.D.'s and Heather Chandler's cases.  Secondly, they both seem like the epitome of cool from the outset ("you wanted to be a part of the most popular clique in school") and it's up to Veronica to peel back their facades to expose the true monsters beneath.  The similarities between Heather and J.D. can be viewed best through Veronica's eyes.  We first meet Veronica after she has already grown sick of the Heathers and their antics (Chandler most of all).  What we gather, though, is that there was a point prior to the film where she was blinded by the Heathers' popularity much like the rest of Westerburg is. ("Betty Finn was a true friend and I sold her out to a bunch of swatchdogs and diet Coke heads").  After the disastrous Remmington party Veronica "wants to kill" and dreams of a "world without Heather" where she is free and can regain the part of herself she lost when she joined the clique. 
"Dreadful etiquette, I apologize." 
On cue, like some sort of deranged Romeo,  J.D. comes onto the scene to become the "world without Heather."  In the love scene on the croquet court he defines such a world ("Heather Chandler is one bitch that deserves to die.") and the very next morning he creates it.  No longer does Veronica have to deal with the "cool" monster that was Heather Chandler (despite the fact that Heather Duke rises to fill her place) she now has J.D. to contend with.  And even though Heather seemed like the very definition of the  kind of person that makes Westerburg (and society in general) a bad place to be;  J.D. is even worse.  He's the sociopath, the aggressively antisocial psycho, the murderer.  He's not the best element for society either.  
"Cool guys like you out of my life."
 It's seems to be Veronica's curse that she discovers the true faces of  J.D. and Heather Chandler too late.  She longed to latch on to both of them ("And to think there was a time when I actually thought you were cool!") and afterwards there's nothing she seems to want more than to get both of them out of her life.  ("I'm not trying to bring anyone back, except maybe myself.")  We have to assume that she is finally free after J.D. blows up and we see her take the red scrunchy out of Heather Duke's hair and wrap it in her own.  Veronica has decided to no longer be the lamb following the cool shepherd around the hallways of Westerburg, she takes charge of her own life and whatever will be will be. 
"Cause nobody loves me!"
 Sympathy for the Devil.  It seems cliché for J.D. to say that no one loves him in his final anit-society speech.  Is it a lame attempt to provide reason to his psychosis?  Or is it the only glimpse of his own despair we truly get to see?  He's on his knees bleeding and ranting, adamant about not telling Veronica how to disable the bomb, and he hits us with this almost pathetic cry for help after he had exclaimed earlier on: "Strength damnit!"  If we take his confession as a serious one: that he's lonely, that he believes no one loves him, that he is despairing because of his feelings; then we can see the knowledge that J.D. has gained when he decides to kill himself.  No matter how terrible Heather, Kurt and Ram were to the people around them, committing suicide drew pity, reverence and love to them with magnetic force.  If J.D. wants to be loved, what better way to do it than to commit suicide?!  (I'm not saying to go out and commit suicide).  Heathers' black-comedic theme centers around this half-true misconception of suicide in general.  Even J.D. figures it out.  Instead of killing everyone else, and becoming the ultimate villain, he realizes that he'd rather kill himself and receive the same kind of positive attention that his victims did.   
J.D.'s desire to clean the slate is an example of his dissatisfaction with his life. It's another connection to Heather Chandler.  Heather is not happy with her life either.  Even though the concept is deliberately placed in her mouth by J.D. and Veronica's forged suicide note, and later on comes up again in Ms. "Phlegm's" class, the idea that is most easily seen in the Remmington sequence.  Again, it's subtle; but during the frat party we see Heather reluctantly give head to David the college guy, and then spit at her reflection in a bathroom mirror.  Like J.D.'s confession in the boiler room, it's the only glimpse we have of Heather's own despair.  As paradoxical as it seems there are similarities between J.D. and Heather.  Between the murderer and his victim.  Between the character in the movie we love and the character we hate.  How ironic. 
--Elisa Higgins, 1999
  
 
 
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