There's a big difference between reading about something that happened in history books and watching it done right in front of your eyes.
I'm sure a LOT of people would be happy to read that Red Skull was killed in the newspaper. But a lot of those same people would likely be sicked at the sight of seeing his head crushed by a cinder block.
It's the same way people have no issues buying a bucket of KFC, but would likely be sicked at the sight of a live chicken being processed to be eaten.
Murder incites a strong emotional reaction, even against a person that arguably deserves it. Heroes in particular... that's nothing new.