You allege that the Disney/Marvel movie The Avengers is some way insults people what were adopted. You give a partial quote to the scene, only the lines that contribute to your "cause". The full quote is as follows:
Bruce Banner: I don't think we should be focusing on Loki. That guy's brain is a bag full of cats. You can smell crazy on him.
Thor: I care how you speak. Loki is beyond reason, but he is of Asgard. And he is my brother.
Natasha Romanoff: He killed eighty people in two days.
Thor: He's adopted.

As you can see with the extra context Thor was defending Loki, then when called on it, when confronted with the fact that he just associated himself with this evil being, he then distances himself by saying Loki is adopted. He is not saying being adopted is bad, he is not saying that being adopted makes you evil. He is only separating himself in some way, from Loki himself.
Proposed change: Jamie Berke stop finding hate where hate does not exist, and moreso, stop spreading that hate to others who don't know any better.

Link to the original petition: http://www.change.org/petitions/marvel-comics-marvel-comics-apologize-to-adoption-community