Warning: SPOILERS for Only Murders In the Building episode 9, "Double Time."
Only Murders In The Building seemed to have solved its central mystery, with Charles-Haden Savage's new girlfriend Jan being fingered as Tim Kono's killer, but a consideration of the evidence suggests there may be a third killer at work in the Arconia apartment building. Moreover, this killer seems to be stalking Jan for reasons unrelated to the main murder mystery.
The Only Murders in The Building story centered around the violent death of Tim Kono (Julian Cihi), a young financial advisor who lived in the same building as out-of-work actor Charles-Haden Savage (Steve Martin), musical director Oliver Putnam (Martin Short) and aspiring artist Mabel Mora (Selena Gomez). United by a love of true crime podcasts, the trio started their own podcast discussing their investigation of Kono's murder, unwittingly exposing a grave robbing enterprise organized by deli magnate Teddy Dimas (Nathan Lane) and solving the murder of Mabel's friend Zoe by Dimas' son. Unfortunately, they were no closer to finding Tim Kono's killer, who apparently stabbed Savage's new girlfriend, Jan (Amy Ryan), in retaliation for their sleuthing.
As a result of the attack on Jan, Only Murders In The Building episode 9, "Double Time," found Charles-Haden Savage ready to give up the game and focus on building a new life with Jan as she recovered from her wound, as Oliver and Mable continued the investigation. In addition to discovering that the same poison that killed Tim Kono was also used on Evelyn, a local cat who died mysteriously on the same day, they learned that Tim Kono had apparently been romantically involved with a woman who was noisy in bed, according to his next door neighbor. This led them to examine the sex toys they had retrieved from Tim Kono's apartment and the discovery that one of items wasn't a sex toy, but a swab specifically designed for cleaning bassoons.
With Jan being a professional bassoon player, this would seem to be a clear indication that she is Only Murders In The Building's main killer. Another interesting point toward that end was Charles-Haden Savage's discovery in "Double Time" that Jan had lied to him about being first chair in her orchestra, suggesting she could easily be lying about other details of her life. It would also explain why she tried to integrate herself into the trio and steer their investigation towards Evelyn's owner, Howard Morris. However, there is one clue which hinted that Jan is not Tim Kono's killer; the threatening note she received in Only Murders In The Building episode 8, "Fan Fiction."
While it's plausible that a killer might stab themselves to eliminate themselves as a suspect or fabricate a threatening note, there would be no reason for Only Murders In The Building to show Jan, who was alone at the time, discovering the threatening note on her front door if she had planted the note herself. This would defy all the standard mystery conventions and be considered a cheat. While it's not implausible the writers of Only Murders In The Building might fake out the audience in this manner, it seems more likely that there's another would-be killer in the Arconia, who is stalking Jan for their own reasons.