“These… look like regular people. They can blend in. And all they got to do to kill you is touch you. Their toxins get in your system, all of a sudden you’re hallucinating your worst nightmares, and pretty soon you O.D.”
It’s really interesting and tragic if you take a look at all the instances the show not only used nightmares, hallucinations or false memories to explore and frame Dean as an unreliable narrator, but also to delve into Dean’s psyche and with that biggest fear: not being able to save or protect the people he cares about most and with that feeling like a failure as a friend, brother or partner.
It’s been explored numerous times from 5x11 “Sam, Interrupted”, in which his alcoholism and depression gets explored in the framework of the apocalypse and him thinking he has to save every single person on the planet to 6x01 “Exile on Main Street”, in which he sees himself fail to protect Lisa and Ben from Yellow Eyes to 8x07 “A Little Slice of Kevin”, where we learn that Dean thinks he failed at saving Cas lives and made up a false memory, because he couldn’t deal with Cas staying back to 8x20 “Pacman Fever”, where he enters Charlie’s nightmare, but finds his own with Sam in a hospital bed dying (the same way we find him in 9x01 - A+ foreshadowing once again).
Now in season 10 and particularly 10x09 “The Things We Left Behind” - to say it with Soulless!Sam’s words from S6 above - we see Dean poisoned by the MoC, he is hallucinating his worst nightmares to the point they become real seemingly without Dean realizing it while it happens and well… it has always been just a matter of time before Dean will OD (and given the preview it seems he is pretty close to the edge at this point).
All that being said. The biggest and most tragic difference to me between all the instances before and this last one from S10 is that Dean’s biggest fear is no longer not being able to protect and save his loved ones, to lose them and ending up alone, but himself being the one to bring that on, himself being the danger (as Walter White/Heisenberg would say).
In S3 he already said to his demonic version with snark that “he is his own worst nightmare” and while sadly for Dean it has always been true, that tragic flaw of him has reached new heights with the last episode. And aside from that it absolutely might explain - if Dean is just a tiny bit like Cain - why Cain chose to live alone, far away from any humans…