Warning: SPOILERS ahead for Spiral: From The Book Of Saw.
Spiral: From The Book Of Saw introduces a new Jigsaw killer, but signposts who the culprit is far too early. Tobin Bell's John Kramer served as the villain of the first three Saw movies. Kramer kidnapped victims he felt were undeserving or ungrateful for their lives and forced them to play lethal games that would prove if they had the will or instinct to survive. Kramer himself is revealed in the first movie to have terminal cancer, and by the end of Saw III, he's definitively dead.
The trouble was Tobin Bell's Kramer/Jigsaw was simply too iconic, so later sequels had to find increasingly elaborate ways to keep the character involved. This usually involved elaborate flashbacks, with 2017's Jigsaw even revealing he had a previously unmentioned apprentice named Logan who seemed destined to pick up his mantle. Jigsaw wasn't much of a success though, and it was later announced Chris Rock had conceived and would star as a cop called Detective Banks in a spinoff called Spiral: From The Book Of Saw.
Spiral introduced a new Jigsaw copycat who is targeting corrupt cops in an increasingly elaborate series of torture games. While the spinoff was designed to reinvent the long-running saga, it ultimately proved disappointing to many. The film suffered from a cliched screenplay that felt like a generic cop thriller with some Saw-inspired traps thrown in. It didn't help it had a lack of suspects concerning the killer's identity, and in the first act, it drops an obvious clue that Banks' new partner William Schenk is the new Jigsaw.
The scene is only a half hour into Spiral, where Banks, Schenk and the other detectives are looking over evidence. Schenk takes a break and declares his phone battery is dead, before asking to borrow Banks' phone, who hands it to him but asks he doesn't kill his battery "...watching Twilight." While the movie tries to throw this moment away, it still sticks out as odd Schenk would ask for Banks' phone, and it's later revealed he used it to lure Banks' father Marcus (Samuel L. Jackson) into a trap.
Spiral all but confirms this when Schenk is suddenly "killed" in an offscreen trap about an hour in. It appears the killer captured Schenk and skinned him alive offscreen. The issue is the movie tries to glide over the details, and while there are gruesome flashbacks to a body being skinned, Schenk's face is never actually shown. The fact his "demise" is so abrupt also stands out, so when he finally emerges as the mastermind before Banks' final test, both the mobile phone moment and the skinning scene all but confirm he's the killer long before the story reveals it.
While Spiral: From The Book Of Saw genuinely tries to offer a fresh take on the Saw movies, the story needed another pass. Even if the phone moment was cut, there's a genuine lack of suspects or other red herrings to point away from Schenk. He's the most obvious suspect from an early point, and after the phone incident, viewers have to watch Rock's Banks chase his own tail for nearly an hour before the truth catches up to him.