How is Sanders the "end of rational" Democrats when he is basically a European leftist?
Because he's an ideologue.
If Bernie Sanders and his campaign weren't running their own little inquisition I'd have a mild preference for Hillary Clinton because she's the more intelligent, more skilled candidate. But Sanders would be a worthwhile alternative because his policies are more or less on the right moral side of the argument.
His methods on the other hand are entirely deplorable. He's been attacking Clinton this entire primary with an entirely disingenuous methodology because she uses the political finance systems build specifically to attack her to her advantage. He attacks DWS because she has historic ties to Clinton. Now he's attacking Frank and Malloy because they've supported Clinton and HIV/AIDS activists because they didn't like being used as a PR item by his campaign.
Obama came into office with the concept of never sacrificing the good in search of the great. Bernie Sanders demands everyone sacrifice any alternative theories in pursuit of his ideology, and woe to the person who actually fact checks or questions him on how exactly those ideas would turn into real, effective, legislation.
This is exactly what the Tea Party was and how we've wound up with a Republican party in 100% lockstep on climate change, worker's rights, healthcare, etc.. If you don't get in line the Tea Party and company would purity test you the fuck out office in the primary and baring that would ruin your chances of winning the general. Sound familiar? Just because the basic concept is good doesn't mean it excuses the methods because enabling those methods will ultimately lead to someone using them for something far less acceptable (see Donald Trump and the very real possibility of the DNC dealing with an anti-vax, anti-GMO, anti-nuclear power candidate in the very near future in Sanders' wake).