"Never interrupt your enemy when he is making a mistake". Do the protesters think that Trump has a significant shot at winning the General? In which case they must attempt to deny him the nomination. If they don't then either they're being counter-intuitive or fail to see the wider picture - or they're just wasting their time. It'd be interesting to see if they think he has a chance.
Now, Hitler comparisons are stretched at best, honestly I see enough people compare British Conservatives to Nazis that in almost any case it peeves me because it's almost always unreasonable. Less so with Trump than others I guess, but a stretch nonetheless. As has been said before Berlusconi is a far more reasonable comparison, accurate I'd say.
As for the General election, if it's Hillary v. Trump it's a lose-lose. Trump I'm not going to attempt to accurately predict, because if he know one thing with Trump it's that he's erratic. We can gauge the general course, and I'm sure people can comment on that, but it's the style of his administration that will be interesting to see. As for Hillary, a lot of people like to call Sanders utopian. If anyone's utopian it's Hillary. To think you can just carry on as things are (near-enough) and it'll all turn out fine. "The cracks aren't there if I don't look at them". Yeah, that'll turn out well. What on earth the situation will be like in 2020 I dread to think.