Some Sanders supporters don't see the difference between Trump and Clinton. Remember the Nader supporters who didn't see the difference between Gore and Bush back in 2000? That worked out well.
I used to think that at least people who made poor choices would learn from them but now realize that the feedback comes so long after their action that they don't. The same progressives outraged at Iraq and the de-regulation that helped to make the 2008 recession a Great Recession aren't clever enough to see a link between those outcomes and their votes in 2000. So they're at it again because one thing they do have in common with Trump supporters is a hatred for the current system and a willingness to crash it if it won't change to accommodate their personal views.
Still, if the Republican Party is dumb enough to nominate Trump ,and Sanders supporters are deluded enough not to see a difference between Clinton and Trump, the country likely deserves what would follow from a Trump reign. Still, it's a shame for the rest of the world that has yet to recover from our last major failures.