I've seen stuff recently about Carter and Spotnitz intentionally writing Diana as opaque and "mysterious" so you couldn't tell what her motives were, and also that she was there for Scully jealousy/Mulder distraction yeah. That's kind of what happened, except that by combining those things they made it so that the fans wouldn't actually ever believe she was trying to help Mulder For Real. Because of the Other Woman shtick.
It's so frustrating to me because intentionally under-writing her to keep Diana mysterious was such a giant waste of having Mimi Rogers guest. Who is a much better, more powerful and subtle actress than most of their 'baddie' stable, save for Laurie Holden/Marita Covarubias.
It's also frustrating because I've believed all along that she wasn't this malicious man-hunter mata hari trope thing the way she gets portrayed in fic. Like, the same fans who are always grumbling about how Mulder's this selfish ass who's actually just annoying and not that great and Scully deserves so much better, etc, BUT also think this powerful Consortium lady, Diana, is going to ruin her whole life and her alliance with CGB Spender in order to have an affair with Douchbag Mulder?
Anyway, using in-universe logic, looking at all the facts about The X Files reality that the Consortium/Syndicate knew that Mulder and Scully didn't, and looking at the hold over Diana that CGB had, and her ultimate end.... Yes, I do think she was genuinely trying to help Mulder (and Scully and the rest of the world by extension).
Diana was just coming at it from the other side, like so many of their informants. She knew that the aliens and the invasion threat was real, and had known for years. Maybe when she and Mulder were working together the first time and maybe not. We know she studied and worked in "parascience" and intelligence which means she probably got funneled into the conspiracy group's clutches early in her career. Just like their other informants, it seems like she believed in the necessity of the Consortium-Syndicate's work, but not all of their methods and endgame plans. Like Krycek and Ronald-Deep Throat and others, it seems like she felt Mulder was a source for good and someone to be protected, and someone who could force the Syndicate-Consortium to change their direction on some projects.
It also seems like she was not that directly involved in the actual projects until after she was pulled away from Mulder "to work in counter-terrorism" some amount of time pre-canon, and seemingly did more legitimate work with the paranormal before that. She and Mulder worked together "sometimes" but no i don't think they were ever partners or that Diana ever worked directly on the X Files before Mulder's demotion in S6, because if she had, they would have said so.
Also because, in-universe, CGB Spender wouldn't have risked compromising Diana as a future asset so early in the game.
Her brief appearances all indicated that though she remained allied with people like CGB Spender through the utility of 'oppressive alien forces are coming, someone has to do something,' she like Mulder personally and thought his motives were good. She didn't want to see him killed in the apparently-imminent invasion or the internal civil war of the Consortium and the greys vs rebels, and even though she was willing to co-opt Mulder and abuse his boundaries in a vulnerable state (and I do sincerely believe that the writers didn't intend or understand the consent violations they were implying because they are consistently profoundly Bad On Those Issues), she wasn't willing to see Mulder die by CGB Spender's ad-hoc brain surgery experiment. She knew she'd be killed for betraying Spender and having his son taken away, but she still tipped Scully off.
Also.... people feel what they feel, and it is what it is. But it does bother me that objectively, Alex Krycek and Diana Fowley are equal as romantic-sexual threats to Scully's place in Mulder's life. Both have feelings for him, and there is chemistry there in both cases. But Mulder never does anything to show he'd choose anyone but Scully at any point. Objectively, Krycek has done much more to harm Mulder and Scully, individually and together. But Krycek is 'Haha that beloved Baddie, the rat can survive anything, look at him go <3' territory in fandom discourse, while Diana is 'ewwww, that awful woman, i hope she suffered!' territory.
Like, no, you don't have to like and have empathy for the bad guys, but that dichotomy of reactions is something to pay some amount of attention to. At the very least it worth looking at the text again to see where your feelings are coming from and what they're based on, imo.
Anyway. Killing off Diana and/or not fleshing her out more while she was there was a waste. (The same goes for Cassandra and Jeffrey Spender. There would have been a lot more places to go with the story and Mulder's understanding of his past if all three of those characters had been around at least until the Samantha story found a conclusion.)