One of six things could be the answer to the issue at hand. The first of which is that since the Mark of Cain caused Dean to become a demon, maybe since Dean still had the mark, he was never fully cured. The second potential solution is that since San had given up on the trials and stopped suffering the effects, the trials pretty much reset. Third is that Dean never lost being a demon, and that he was just suffering from the effects of the mark that it was just temporarily suppressed. This could mean that his demonic presence was tied to the mark. This may explain why he couldn't bail in the form of smoke. Fourth is that the mark was keeping him alive, not the fact that he was a "demon", which leads me to believe that Dean may never have straight up been an actual demon. Fifth is that the necessary steps were not taken in order to properly cure Dean. Lastly is that it wasn't exclusively Sam that cured Dean, and that Castiel may have also cured Dean, which may leave the third step incomplete, seeing as how Sam wasn't the one who actually cured him.
- Since the Mark of Cain caused Dean to become a demon, maybe since Dean still had the mark, he was never fully cured.
- Since Sam had given up on the trials and stopped suffering the effects, the trials pretty much reset.
- Dean never lost being a demon, and that he was just suffering from the effects of the mark that it was just temporarily suppressed. This could mean that his demonic presence was tied to the mark. This may explain why he couldn't bail in the form of smoke.
- The mark was keeping him alive, not the fact that he was a "demon", which leads me to believe that Dean may never have straight up been an actual demon.
- The necessary steps were not taken in order to properly cure Dean.
- It wasn't exclusively Sam that cured Dean, and that Castiel may have also cured Dean, which may leave the third step incomplete, seeing as how Sam wasn't the one who actually cured him.
There may be something else that could explain it, but in the season 10 midseason finale, the effects of the mark resurface, but in the season 10 finale, Dean doesn't kill Sam, showing that he is not completely evil/demonic.