Bernie supporting Hillary would be him betraying all of his promises and ideals, and going back on all of his words and pretty speeches. Do you honestly think people will do any of what he says if he disappoints them like that? Hillary is the most hated Democratic candidate in history, and if Bernie supporters get pissed off at Bernie - which they absolutely, undeniably will if he starts campaigning for Hillary - then they'll absolutely stay at home, or even more of them will go out and vote for Trump out of spite.
Bernie campaigning for Hillary would be the dumbest thing he could ever do, and if he ends his political career supporting a complete corporate whore and monster of a human being, then good riddance.