In "Sitzprobe," though, Loretta confesses to poisoning Ben's protein shake and later pushing him down the elevator shaft when she realizes the police are suspicious of Dickie.

Source: https://www.popsugar.com/entertainment/only-murders-in-the-building-loretta-theory-49276258

That he does, his body falling from a broken elevator shaft, just as the true crime trio exits to find it.

Source: https://www.digitaltrends.com/movies/only-murders-in-the-building-season-3-ending-explained/

While Donna confessed to attempting to poison Ben to improve the reviews for Oliver's play, she later also admitted that she pushed him down the elevator shaft.

Source: https://screenrant.com/only-murders-in-the-building-season-3-ending-explained/