I think he is acting in the best interest of the country, or at least he believes he is. If he is fundamentally against what Hillary stands for then why should he stand down, regardless of his chances of winning. Just because Hillary decided to stand down to Obama and not stand behind her convictions does not mean that Bernie should.
There isn’t a party to unite because Bernie supporters feel the same way he does. We fundamentally disagree with the direction of the party and at the moment the head of that party is Hillary. Either the party changes or a new party is created, this isn’t a unprecedented idea and has happened throughout US history when a party gets off track, or in this case shifts to the right center track.
I don’t want to see Trump as president but I don’t want to see Hillary as president either and I am not going to vote against my conscious to select the lesser of two evils.