He backed Clinton because he considered Trump a threat to democracy. He was afraid, and still is, that Trump would/will undermine the democratic mechanisms of the US so that he would effectively have a dictatorship. For someone who calls himself a classical liberal and cultural libertarian, you'd have to be a fool to support Trump. Bernie is right, Trump is a threat to liberal democracy (wherein liberal is the classical sense like you believe in). And it took incredible humility to support Clinton after she cheated him like that. He didn't make a stink because he knew what was at risk, and made a very principled sacrifice, and here you shit on him for "selling out". He made it very clear in the primaries what the difference between him and Clinton was and why he didn't like her, but swallowed his pride and supported the side that wasn't a threat to liberal democracy worldwide. To be able to do that, he had to have more humility in his left pinky toe than the entire Clinton family put together has. And he has said all sorts of things about how he would make a better President, including dealing with automation, showing that he's thinking ahead just like Yang is.
I don't support Trump. But I did (and still do) think he's the lesser of evils between him and warmongering neolib/neocon Clinton, if only slightly.
Bernie was just as unlike Hillary Clinton as he was unlike Trump, maybe more. They actually both had the overlap (at least initially) of being anti war, anti free trade, bringing manufacturing jobs back to America, etc.
There are many things about Trump I dislike, but I disagree with the laughable notion that he's a "threat to democracy". This is largely just media hype and sensationalism by Dem politicians and the media, who's interests quite obviously conflicted with an outsider like Trump.
I still believe Bernie would have beaten Trump because he had the Rust Belt/Mid West of the US (trust me, as a Midwesterner I know - our jobs have been hit the hardest). Clinton did not have these regions - many of which were the swing states. Hence why she lost. If anything, it was Clinton that should have backed down, not Bernie.
I agree as a fellow midwesterner that Bernie would have beaten Trump, largely through flipping the midwest, particularly PA, MI, and WI. Indeed, he'd have crushed him. I also agree that Clinton could never have won them, based on what I saw in the region, and that since she was complicit in organizing the DNC rigging, she should have backed down. And yes, she's a neolib/neocon warmongerer.
But my fears regarding democracy as a whole are altogether different from the media hype, and they're coming true. The media hypes it as "Oh sweet Jesus he said some mean things it's the end of the world" when in actuality that has different consequences that are also bad for society but aren't the threat to democracy. The media hypes it as "Oh the humanity he rolled back such and such right" and rolling back rights is not cool to be sure but Republican presidents have done it before and had their rolling back undone. Indeed, Democrats have done it before (though much longer ago) and eventually rights win the day. And perhaps what I refer to, you also think is media hype, but my thought process is more complex than simply "orange man bad". I too am anti-war, I too am skeptical of free trade, and I too want to support blue collar workers and jobs. However, Trump is none of those things, he just pretends to be when it's convenient.
When I say he's a threat to democracy, I mean that it sure as hell looks like Russia tried to influence our elections, even our own intelligence agencies staffed by Trump come to the conclusion that Russia tried to influence the election, yet rather than act like an innocent person and encourage the investigation into whether he worked with them to do so, he tries to obstruct it at every turn and insist that Russia is innocent against the advice of his own generals and national security staff. His refusal to cooperate with the investigation makes him look guilty as hell, and his refusal to take action to protect our democratic mechanisms from foreign interference (be that Russian or otherwise, I mean there was a time when Saudia Arabia loved Clinton and probably would have helped her if they had the chance, not to mention China is developing the capabilities and there's no telling what they'll do), well that is a threat to my vote being properly counted, and that's about the most fundamental threat to democracy you can get beyond declaring martial law or staging a coup.
When I say he's a threat to liberal democracy everywhere, I don't just mean in the states, I refer to his tendency to prop up the ambitions of dictators and bully our democratic allies. How the hell do you get off defending the dictator king of Saudi Arabia when he literally invades foreign sovereign soil and kills a journalist?! Yet that's exactly what Trump did with Khashoggi. That's a double threat, both to freedom of the press, in that it normalizes killing journalists if an allied government of the US can get away with it with no consequences, and to the ability of the international community to punish human rights abuses, in that there was broad international consensus that Saudi Arabia needed to be punished yet it didn't happen. Only Canada had the guts to do anything about it and we sat there and let Saudi Arabia bully Canada, one of our closest allies, for daring to cross them. Our other democratic allies wanted to do something but didn't want to be bullied like Canada was and without the support of a major world power like the US, it's hard to support human rights and the rule of law on an international scale like that.
But perhaps none of this implicates Trump directly enough in an immediate assault on democracy in America for you to give a shit? Well how about his stupid national emergency declaration? You had some of the most conservative Republicans in the Senate opposing it because it's a blatant breach of the most straightforward interpretation you could possibly have of the U.S. Constitution. He's trying to consolidate power into the executive branch in a way no president ever has. People act like Obama was horrible in this regard because of his executive actions, but none of those executive actions, or those of presidents before him, ever had so much open disregard for fundamental Constitutional separations of powers as that emergency declaration did. Trump has opened a portal to hell that needs to be shut quickly. Today it's his pointless, ineffectual border wall wasting money we don't really have, tomorrow with a future president it'll be climate change prevention (though that wouldn't be so bad, but it should happen through the proper legislative channels), then next thing you know some president will seize money to give out reparations for slavery, or combine the unconstitutionally seized power of the purse with Congress' precariously maintained power to declare war (that's actually all on Congress, not Trump's fault at all) and declare and fund a war all of his own! Again, not necessarily Trump, as the guy won't live forever even if he managed to completely rig our democracy to let himself rule until he dies, but this is about more than Trump, it's about the fundamental institutions of democracy being damaged by his complete lack of regard for democratic norms and the rule of law, and what that might mean for our future.
You can disagree, but you have to at least admit there's a rational train of thought here that goes beyond emotional reaction to media sensationalizing. It's that rational train of thought that Bernie is following when he opposes Trump. As for Bernie talking about Trump already, well that's less him being Trump obsessed and more a projection of confidence that he'll win the primary, as well as a signal to the powers that be that no matter what, he's already decided to oppose Trump, in hopes that they don't see him as a spoiler that needs to be attacked. By running against Trump now, Democrats trust him more even though he's an Independent, and he also has the opportunity to weaken Trump ahead of time by highlighting his failures so that when he wins the primary Trump is already partially weakened in the general. Bernie has talked plenty about what he plans to do, in fact through constant campaigning since his primary loss in 2016 he's redefined what all the other Democrats are planning to do. Medicare For All was supposed to be a Bernie 2nd term goal, only possible after working at convincing the American people for 4 years to trust him and believe in his vision even for his bigger asks like that. Yet now he has most of the nation ready and supportive of Medicare For All, and it would easily be a 1st term goal for him, and his plan blows the other Dems corporate focused Medicare For All plans out of the water. His plan is the only one that simultaneously obsoletes almost all private insurance companies while saving trillions of dollars over our current system, and that's according to the numbers of a Koch brothers funded study, so you know they tried to weight the numbers against Bernie's plan.