Warning: This article contains SPOILERS for Arrested Development season 5.
Arrested Development season 5 has largely revolved around the mystery murder of Lucille 2, and the season finale reveals her killer to be none other than her former lover, Buster Bluth. The shocking twist is revealed in the final moments of the season finale, in what is likely the darkest plot point in Arrested Development's history.
Arrested Development's polarizing season 4 (the first produced for Netflix) spent much of its time creating reasons for virtually every member of the Bluth family to want Lucille Austero - colloquially referred to as Lucille 2 in deference to Lucille Bluth - out of the picture, and the season 4 finale ended with the vertigo-prone socialite disappearing on Cinco De Quatro. Buster - the family simpleton who shares a deeply unhealthy relationship with his mother, Lucille 1 - was immediately a top suspect, owing to his rocky romance with Lucille 2 and the antagonistic relationship she had with his mother. But Arrested Development leaned so hard into the idea that Buster was the likely killer during the first half of season 5 that it all felt like a red herring, yet another case of a Bluth family member getting caught up in a massive misunderstanding.
This time, however, there was no red herring. After escaping the murder charges through a wildly improbable mistrial, Buster agrees to be Gob's assistant for his magic trick at the unveiling of the Bluth-backed Mexican border wall. Gob's trick involves him seemingly being submerged in cement and trapped in the wall, only for him to appear unscathed, alongside the presumed dead Tony Wonder, who had really been ducking the threats of the gay mafia. The plans for the wall, of course, fell through, which was all part of an elaborate scheme to get the family out of debt. As the family gathers to celebrate their success, workers begin to disassemble the small portion of the wall that was poured ostensibly with Gob inside. The human body poking out of the wall is presumed by all to be Gob's magic dummy, but when the wall is broken down, Lucille 2's body comes flying out. Buster, unaware the wall would be destroyed, planted the body there. He admits to the family he killed her, as the oblivious Michael and George Michael flee in the stair car, and the rest of the family stands around their homicidal brother, speechless.
Aside from the fact he was blackout drunk on the night of the murder, there were hints along the way that Buster was capable of this. It's revealed through a series of flashbacks - framed as part of the movie Ron Howard is making about the Bluth family - it's revealed that Buster killed his grandmother by pushing her down the stairs when he mistakenly believed she was going to take him away from his mother (she actually wanted Lindsay, revealed to be her long lost biological mother). Lucille put Buster into therapy after the incident, where he was convinced that he was actually covering for Lucille killing his grandmother, which explains more than a little about the adult Buster's eccentricities.
Arrested Development doesn't get into the details of how or why Buster killed Lucille 2 - that would presumably be the driving narrative of a hypothetical Arrested Development season 6 - but there are enough clues to piece together what likely happened. Arrested Development has always traded in deeply flawed, sometimes outright evil characters, but making one of the core members of the family a conscious murderer is the show's darkest plot point by a fair margin. If season 5 winds up as the end of Arrested Development, this would be a shocking note to go out on.