26 years ago - Frances Kane is born.
12 years ago - 14-year-old Frances's powers manifest, accidentally killing her father & brother. She is raised by her mother who secretly hates her.
9 years ago - 17-year-old Frances goes to college, where she meets Wally West, who nicknames her Frankie while they date.
7 years ago - 19-year-old Frankie begins working with the Rogues. Her powers overwhelm her, allowing her Magenta persona to take over. Wally West brings her in to get her the treatment she needs, and ends the deal with the Rogues, bringing them in.
3 years ago - 23-year-old Frankie is freed from her treatment facility by Clifford Devoe to join his new team of Rogues. they are stopped by Wally West & the original Rogues, and she is taken again taken into custody, this time by Amanda Waller.
2 years ago - 24-year-old Frankie is a part of the new Suicide Squad, but she is tragically killed on her first mission by Atomic Skull.
Magenta's Comic History
Frankie first appeared was back in 1982 as Wally West's girlfriend in the pages of the New Teen Titans. It was a weird era for Wally... he was still Kid Flash but was constantly being depicted as being in a transitional period, trying to retire from the role and go to college, not an active part of the current Titans. Frankie was a supporting character for what was basically also a supporting character.
Wally took over as the Flash in 1985, and suddenly his role in the DC universe got much broader, but also much more complicated. Wally West's place in the world was still being fleshed out. The idea of just how we were going to be depicting Wally as an adult was still being worked out, and wouldn't really solidify until Mark Waid started writing him in 1990.
While the idea that Frankie had magnetic powers was actually introduced back in her early appearances, what we saw through all of this was a constant reference to her diminishing mental state. I don't believe there was any moment when it was decided that this fairly innocuous supporting character was going to shift into a villain. Rather she seemed to devolve over time, until she finally developed this new evil 'Magenta' personality, which has defined her ever sense.
Inherent Misogyny in Magenta
While I truly don't believe anyone intended for the character of Frankie Kane to devolve into a villainous depiction of a bipolar ex-girlfriend, the fact is there is a pretty frustrating trend that occurs in comics (and other media for that matter). Female characters are constantly being written as the main characters girlfriend or love interest, a role which doesn't really give them a lot of room for their own personality or character growth, and as a result the character winds up being pretty bland or uninteresting, which leaves them ripe for being turned into the subject of all sorts of unplesantness.
The most infameous trope to come from this is 'fridging', when a female character is killed off just to support the male character's story, but there a lot of other ways this can happen. One of them is for that character to be depicted as going insane and becoming a villain. It's happened to Green Lantern's longtime love interest Carol Ferris, it happened in the pages of the X-Men to Madelyne Pryor... and it happened Frankie.
You don't generally see this sort of trope happening that often anymore; I think most writers are a bit more self-aware when it comes to their blindspots. We still have these legacy characters. though, and there really is no way to include someone like Magenta without at least some level of that old trope in her. We just need to make sure we acknowledge this sort of thing.
Our Magenta Story
Whatever else Magenta might be, she is a really great part of the Flash's Rogue's gallery. Even once we've taken out a lot of the ideas about her eventual mental break being the result of Wally's attention or lack thereof, she's still a tragic part of his past, and that gives her a lot of dramatic weight. Also, let's not forget that mastery of magnetism is an INSANELY scary power for a bad guy to have. Frankie is actually a really great example of this sort of power being handled well. She's as much a victim here as anyone, even as she's weilding this huge level of power. The character becomes a real dramatic powerhouse.
We've really limited her use overall, because a character like this loses a lot of their potency the longer she sticks around, eventually becoming just another villain among many. We've given her her original descent into villainy (making it the catalyst for Wally finally deciding to break up the Rogues), and then made her a member of the Thinker's New Rogues, specifically doing so because her presense brings so much dramatic weight.
We were building up a story for the Suicide Squad where Waller oversteps her limitations and brings in characters that are too powerful to control and it results in tragedy, and it somehow felt completely correct to have that tragedy be Frankie's death. I almost feel like I owed it to Frankie to somehow give her a redemptive arc, but that's never really been what the character was about.