deconstruction as a way of life

The concept of the predatory lesbian in fiction is not 'there is a fictional lesbian who commits acts of sexual predation and violence in the story'. The concept of the predatory lesbian in fiction is (to summarize extensive better definitions of the term) 'there is a fictional lesbian who commits acts of sexual predation and violence because she is a lesbian in the story'.

The reason this is a bad thing is because in real life, the idea that any 'non-normal' (not straight and cis) sexuality is inherently a cause of potential deviance is bad. One of the key anxieties behind the predatory lesbian as a trope is that if a woman is 'willing' to transgress one 'normal' sexual boundary, she will be willing to transgress every sexual boundary. It's an issue because it posits that 'being a lesbian' is not meaningfully different from, for example, 'being a rapist, zoophile, or necrophile'. It collapses all 'non-normal' sexualities into a single type of indiscriminate deviance and violence.

In real life, this hurts lesbians (and anyone else affected by similar stereotyping, i.e. anyone not straight and cis). It's a justification given for excluding them from social life, for treating them as inherently suspect and criminal, for insisting they can't be trusted around other women, for acts of real physical and sexual violence against them (whether to punish or 'correct' their sexuality), for the criminalization of being a lesbian that still holds true across a large part of the world and is more real in places it's supposedly no longer legally enshrined than you might think. Homophobia as a whole and lesbophobia specifically is partially fuelled by the idea that being a lesbian is to be inherently at higher risk of committing sexual violence.

When an author writes about a lesbian character being overcome by shameful and deviant desires that lead her to sexually assault another woman, it's not theoretically impossible to write this with sensitivity to the stereotype. It is, however, extremely difficult to pull off. It's also something that people should accept will come with a great deal of criticism because of the biases that it taps into.

It's also not transgressive. The concept of a predatory lesbian is extremely normalized. Ask any given set of lesbians about how group changing rooms or friendships with straight women tend to go for them, and you'll find this out. It's an old, and until recently broadly and openly accepted everywhere, trope. Even now, it just tends to be more covertly phrased by the people who believe it but have an idea that overt lesbophobia isn't as condoned as it used to be in some places, and it still gets voiced loudly and proudly by a solid proportion of the world at large.