Here's what we know:
* Jason's death is shrouded in mystery: The circumstances surrounding his death are murky. We only see his body being pulled from the lake, and there's no clear explanation for how he ended up there.
* The "A" team has a history of violence: Throughout the series, "A" has been shown to be capable of extreme violence and manipulation, so killing Jason wouldn't be out of character.
* Red Herrings and Misdirection: The show deliberately leaves the viewer with many suspects and red herrings, making it difficult to pinpoint the killer. This includes possibilities like Toby, Melissa, and even Aria's father.
* The "A" team is not a single person: As the series progresses, it is revealed that "A" is actually a team of people, adding to the mystery of who was ultimately responsible for Jason's death.
Ultimately, the show leaves the true killer's identity unresolved. While many fans have theories, the writers never offer a definitive answer. This ambiguity adds to the suspense and leaves viewers speculating long after the series ends.