Surely, in many other books the serial killer is a straight male. Why are their authors not accused of anti-heterosexual speech? Are we promoting some sort of discrimination against straight males? Are they the only ones allowed to be evil?
That's not what OP is referring to. Go read her Twitter feed and blog.
Basically, there's a long and sordid history of the Cross Dressing Predator as a literary and film trope and this has been used against trans people heavily.
Mostly the cultural effects of the Hays code and decades of queer-coding villains as evil or sexually depraved in fiction and popular culture. A fictional male serial killer tends not to simply "have worn a dress," that specific detail, like all details in fiction, is intended to communicate something to the reader/viewer, and most of the time it's to present the character's non-heteronormative behavior as a symptom of their moral evil.
Technically speaking there's nothing wrong with depicting a Black person eating a watermelon and fried chicken but there's context behind the tendency to interpret it other than purely literal terms.
Except the details of being white or male - those are not intended to communicate anything?
Not typically. Being white and male (and straight, and Christian) is considered so normative in Western society and culture that it's basically a blank slate, "default human." How often is a white character's race specifically mentioned, unless it's to form a contrast with some other non-white character's race?
It almost never happens, because white male characters are often just there. Every deviation from that norm has to be specifically mentioned and pointed out, and unless the author is part of the group or culture being portrayed, is usually done through stereotype.