Season 4: Sam Chooses Ruby over Dean, strangulates his own brother into submission, kills and drinks a nurse, breaks the last seal via killing Lilith. Releases Lucifer and starts the apocalypse. Because he thinks that only he can kill Lilith, save the world. Dean tries his best to stop Sam, but fails. (whats with getting choked to half-death by Sam and all that)
Season 5: Sam and Dean are held responsible for starting the apocalypse. Castiel says so, the angels say so, the show says so. Dean accepts that he is indeed responsible and gets to the clean-up work.
Season 10: Sam goes against Dean’s explicit orders and uses witchcraft with the help of Castiel and Rowena. Dean is unaware of Sam’s actions. Sam doesn’t call off his mission even after he learns about the true nature and purpose of the Mark of Cain, which results in the release of the primordial darkness.
Season 11: Sam and the narrative successfully, subtly shift the responsibility on Dean, and blame Dean for the bond he shares with the darkness. Dean, once again, accepts that he is responsible for releasing the darkness, despite his clear innocence in the matter.
Season 11: Dean is kidnapped by the darkness and is unable to contact Sam. Sam goes to cage to visit Lucifer against Dean’s explicit wishes. Because he thinks that he is chosen by God to lock up the darkness. As a result, Lucifer escapes, possessing Castiel.
Season 12: Sam says that “they let Lucifer out” implying Dean’s involvement in what Sam and Cas did. Dean doesn’t contest the truth of this statement and accepts the responsibility for the consequences of Sam’s actions.
As you can see from the above list, this is not an one-off situation. This is a pattern. Sam fucks up, Dean is held responsible for the consequences of Sam’s actions, either overtly or covertly. Sometimes it’s subtle, sometimes it’s blatant –but this blame shifting and gas-lighting definitely happens. Every single season. Also note how Sam’s actions are fueled by his hubris. Again and again, Sam discards Dean’s advice, thinks that he is chosen, thinks that he knows the best –and when Sam’s choices and actions lead to a disastrous outcome like releasing Lucifer or the darkness, when the clean up time comes –then, Sam and the narrative shift the responsibility on Dean.
The cycle continues.