Ok, let's face it, it's the more overused plot of the Avenger : The traitor.

It's kind of funny at this rate, because it's seems that to be an avenger, you must betrayed them first. Let's see :

Wanda (Gone mad and Betrayed them all...killing some in the process.)

Tony Stark (Yeah, You remember, when he betrayed for Kang ?)

Hank Pym (Between Drugs or alcohol, I do know where to start !)

Vision (Isn't the Genocidal robot known as Ultron who's send him to spy them first ?)

The dark avenger. (Okay, it's kind of usurper, but that's just another traitor kind to me...)
The Secret Invasion (THAT was a massive traitorous invasion !)
And the list goes on and on...
That is one of the reason why I didn't really like the avenger. Some of the caracter are great, and it's kind of superhero dream team, but frankly, I found that some plot come back too frequently to be enjoyed.