The focus on May is a red herring. The DNC was strategically undermining his campaign well before that, and we all know it.
As to his being an outsider, the DNC could have forced Bernie to run as an independent but they chose not to. They chose to include him, to give the illusion of healthy competition, rather than risk him splintering the liberal vote as an Independent candidate. Right now he could be throwing his support behind Jill Stein, but instead he is endorsing Hillary and choosing to play the game.