The angry Sanders supporters are just folks who are not threatened or bribed by Hillary—after they learn how evil she is.
Sanders conceded because he was threatened—most likely Hillary told him his family would sleep with the fishes if he did not endorse her.
My theory is that the “Bernie Sanders Campaign” was to him a giant book tour. The guy never had a chance against the combined Democrat power structure. He was a useful foil for Hillary, until he demonstrated her political glass jaw. But Bernie, for whatever reason, chose not to play rough and Hillary was allowed to outpoint him on the Super-delegate scorecard. Bernie knew this would happen if he didn’t get the KO. So either he was threatened very early in this campaign, or he never intended to go all-the-way in the first place. A political last-hurrah for a nobody from Vermont.