@ehBeth,
That's a different argument, better than Snood's, which is that Bernie could run as an independent if he loses the nomination. (this risk seems close to zero)
But your idea (correct me if I'm wrong) is that many of his SUPPORTERS may not vote for Hillary, especially if he hardens his position towards her, going forward. I see some merit to this proposition. For him to keep on fighting until the end in a more and more bitter way could be damaging. But if he keeps it civil and if he attacks Trump, he can help her by staying in the race.
My guess is that he MUST attack Trump if he wants to stay relevant. Didn't he do so already?