Let's be honest, it's not really his responsibility to take care of Hillary's campaign regarding attacks from Trump. He's entitled to put his best foot forward.
It's like saying United and Liverpool are 3 points behind Leicester with 2 games to play. United shouldn't be obliged to just roll over against Leicester in the second last game, to give them a better chance of stopping Liverpool.
If you're in it to win it you're entitled to go for it, it's nobody's fault but Hillary's that she hasn't quite shook him off yet (even if it's pretty certain she'll win).
Actually I admire it, even if it's a bit naive. I certainly prefer it to what you see with the GOP, of 12 guys on stage saying how horrendous the other 11 are until one by one they drop out and preach their love for the last man standing as the one to protect 'American values'. Not quite to the same extent with Trump because he's an outlier, but in 2012 Romney's opponents in the primaries were suddenly proclaiming him as the last chance to overturn Obama's socialist commie republik.
Now I get that that's how party politics works and you have to fight fire with fire and unite against it (otherwise the Dems would be constantly infighting and the GOP would march to victory all the time, though I think the opposite way around was expected this year until Sanders' rise: it was looking like Hillary's procession v a broken GOP) but those partisan politics are for me a big part of the legislative paralysis that's really damaging America.
But anyway, why should Bernie care that someone he's going up against his fighting on 2 fronts? I think that was really an idiotic question to ask if I'm honest. The closest thing to a justification would be 'him stepping out of her way would make it easier for her to fight the big bad Donald' but that makes no sense as A) he believes the figures that show him to be the best candidate to do that and B) he sees her as part of the problems he sees in America anyway.
I'd hardly call that a 'rant' by the way (I only watched the time slot you mentioned so maybe the rest is different) and I think it's quite disingenuous of someone to say it is.
I thought he was correct in what he was saying. He did lose his temper a bit, which accentuated his point.