Detailed Explanation
Rita Bennett Morgan was killed by Arthur Mitchell, the Trinity Killer, in the Season 4 finale "The Getaway." Arthur, a seemingly respectable family man, was actually one of the most prolific serial killers in American history, responsible for over 250 deaths following a cycle of three murders (a young woman in a bathtub, a mother jumping from a building, and a father bludgeoned to death). When Dexter got too close to exposing him, Trinity retaliated by murdering Rita in her bathtub—leaving Dexter's infant son Harrison sitting in her blood, eerily mirroring Dexter's own childhood trauma.