Why is Red Hulk an Avenger? He destroyed S.H.I.E.L.D.'s helicarrier, beat up every superhero that got in his way, and conspired with a Supervillain group to take over the world. He had a Heel–Face Turn, but I don't think that qualify him for being one of "Earth's Mightiest Heroes"
Because the inmates are running the asylum at the moment.
Is General Ross really a villain in the first place? He's always seemed more like a Well-Intentioned Extremist from my point of view.
He's a Hulk soldier without the anger issues, he's a great choice. Besides, he was originally just given missions by Cap in order to make up for all the stuff he did before. After proving to them time and time again that he was geniunely a good guy who would help them save the world, they let him join. I think Fear Itself was the first time he worked on the Avengers team, and that was only because of the chaos going on in NY.
The Avengers have a long history of letting reformed criminals join, going all the way back to Cap's Kooky Quartet. The second lineup of the Avengers, and 3 out of 4 were just reformed supervillians.
Does Spider-man know who Red Hulk is? In Avenging Spider-man he refers to his military background and calls him General! Did I miss something?
Spider-man's Avengers membership no doubt gave him access to the appropriate intel or allowed him to learn it from one of the other Avengers.