The reveal hasn't come in Heroes in Crisis yet, but come on, we already know who it is. It's been in plain sight all along. That's the whole nature of murder mysteries. The plot is always misdirecting us away from the real killer so that when the revelation comes, we all go "Of course! It was so obvious!"
So: Was it Harley Quinn or Booster Gold wot killed 'em all?
Let's see, Harley is a half-reformed supervillain who's insane and has been perfectly comfortable killing people in the past, and we've seen in Heroes in Crisis that she's gone around bonking the other patients at the Sanctuary in the head and killing them. She's the most obvious suspect so of course it's not her, even though we've seen her killing the heroes in flashback, even if they're Booster's flashbacks.
Booster Gold is a walking screw-up who suffers from PTSD as all of Tom King's characters. It seems to be a prerequisite for a Tom King character to suffer from PTDS: Mister Miracle in his recent miniseries, Batman, Gotham Girl and so on. That makes Booster mentally unstable and an unreliable narrator, not to mention a witness lacking credibility on the stand. The story goes to great pains to suggest he blacked out and killed the heroes at Sanctuary.
So of course, if the story is pushing both suspects at us so hard and casting doubt on both of them, because they're both fan favourites – and Harley has her own book and thousands of fans who like to cosplay as her, while Booster used to have his own series – the conclusion is that the killer is NEITHER OF THEM.
This is classic whodunnit misdirection. The killer has been in plain sight all along while our attention has been focused on the main suspects.
The killer is the Sanctuary Artificial Intelligence program that runs the place.
The clues are all there. The AI can create holograms that look like anyone to put the heroes there at ease so they can open up and talk about their trauma. It has a complete database of every character in DC. It controls and runs Sanctuary. It has files on all the patients there and knows what makes them tick, including their weaknesses and how best to kill them.
So all those flashback scenes where Booster saw Harley killing everyone and Harley saw Booster killing everyone are not fake. The AI created holograms to show Booster and Harley want they saw to put suspicion on them both.
Why would the AI, which is supposed to be the nurse and psychological counselor to heroes with PTSD, kill all its patients?
Maybe the Sanctuary AI just got sick and tired of listening to heroes talking endlessly about their hang-ups all the time.
Because AI is evil, of course! Don't you listen to Elon Musk? He believes the Singularity will bring about AIs becoming so much smarter than humans that they would decide to kill all humans. It's the default of pulp fiction, which is what comics is, to portray any new technology as terrifying and evil. It all goes back to Frankenstein.
For positive portrayals of AI, you'd have to go to actual books without pictures, novels by Science Fiction authors like Iain M. Banks who created the Culture, a utopian space civilization run by benevolent and occasionally whimsical AIs. The only recent positive portrayal of an AI in film or TV in my memory has been the Star Trek: Discovery Short Treks episode "Calypso" where the ship's AI forms a friendship with an injured soldier and nurses him back to health before helping him get home.
This is ironic considering AI already permeates our lives in so many invisible ways. It's in the algorithms on Netflix that suggest new shows and movies for us to watch based on what we've seen already. It's on our phones. It's in the self-driving cars being tested that will become widespread in the near-future. It's in medical diagnosis programs that are coming online in the health services. Even as Elon Musk and pop culture keep putting out dire warnings that AI is evil, it's already slowly and quietly taking over.
Meanwhile, we'll just wait for the final issues of Heroes in Crisis to tell us why the AI did it. It was written by Batman, so maybe Batman is just crap at writing AI and should stay away from doing that from now on?