One thing I never hear brought up is the fact the Bernie wasn't even in the Democratic Party until he decided to run for president. He actively ran against democrats and defeated a few in Vermont.
Don't spend a large portion of your career saying you're not a democrat and then feel bad when the party treats like a second class citizen.
Oh he also hooked up with Cornell west a dude basically called Obama an Uncle Tom.
From what I have read, it seems he was on pretty good terms with the DNC during his time in the senate. Even had Obama campaigning for him.