I admit that I'm a Scully/Mulder 'shipper, and I have been practically since the first X-Files episode I ever watched nearly two years ago. Admittedly, that might have something to do with the fact that the episode in question was "Triangle", but that's beside the point. 
There's one thing, though, that I've never really understood about many of my fellow 'shippers, whether they started watching the show during its original run or they became fans of it years after it ended. And that thing is the unusually extreme hatred which many of them direct at the character of Diana Fowley (and, by extension, the actress who plays her). They call her "old"—which shouldn't be an insult anyway, as we all get old if we live long enough. (I wonder how many of them realize that the actress who plays her is only about four years older than DD.) They call her a whore when there's no indication that she sleeps around, whether or not it's for money. They take a grim delight in Scully's report that Diana was found murdered early on in season 7.
Why? As far as I can tell, it's because she's Mulder's ex-girlfriend and possible ex-wife, and she represents the biggest threat of any of "Mulder's women" to the relationship between Mulder and Scully.
To be fair, Diana is a pretty ambiguous character. She left Mulder years ago, but she wants him back and isn't afraid to show it, and she is working for the Smoking Man. She has a way of making Mulder trust her when her actions show that his trust in her isn't completely warranted, and she's unconscionably rude to Scully. But in the end, she does show that she really is on Mulder's side to some degree; she slips Scully clues to Mulder's whereabouts, allowing Scully to rescue Mulder after that experimental brain surgery that Mulder and the Smoking Man undergo after the incident with the spaceship fragments. Her reward? Death. Cue the cheers of lots of Fowley-loathing 'shippers.
But even if she does want to get between Mulder and Scully, the damage that she does to their relationship on her own is almost surprisingly minor. She tries to talk Mulder into admitting that Scully's insistence on proof has been a hinderance, and she fails; he tells her that he's been fine without her. Later, she takes a few jabs at Scully. That's about it. The real damage is done when Scully gets angry or jealous and Mulder gets defensive; they break down their own communication. Scully doesn't trust Diana; Mulder, arrogantly assuming that his past with Diana somehow makes him the expert on All Things Fowley, refuses to listen to Scully or to consider that maybe there's something just a bit fishy going on with Diana. Scully then (understandably, since Mulder refuses to listen) comes to the conclusion that Mulder doesn't really trust her after all, perhaps even thinking that their partnership and friendship didn't mean as much to Mulder as they'd come to mean to her. Is Diana the catalyst? Of course. But she isn't entirely to blame for the difficulty that Scully and Mulder have with each other whenever she comes back into the picture. The biggest problem is with them and the way that they react to her—Scully with suspicion and jealousy, Mulder with defensiveness and a demand for a blind faith that Scully has never been capable of giving.
So, maybe my fellow 'shippers (if they've managed to get through all this without deciding by now to poison my afternoon cup of tea, that is 
) can explain this to me—what's behind all this hatred of Diana Fowley?