3. Thor (Thor, The Avengers, Thor: The Dark World) Reigniting The War with Jötunheim / Failing to Deal With Loki
Never could the saying 'pride comes before a fall' apply more rightly then to the God Of Thunder. His arrogance leads him to Jötunheim, where a quest for foolish vengeance reignites a war with the frost giants that almost costs him his family and all of Asgard. It is only his banishment to Earth that he finally learns some humility, though that is somewhat less evident during his return in The Avengers. It is with his role in The Avengers and Thor: The Dark World where he really drops the ball. His love for his brother, frost giant in blood Loki might be, means Thor does not act decisively and that has consequences. Had he taken his brother back to Asgard before New York, he may have helped to avert the battle with the Chitauri. Even then, when action is taken and Loki returns in chains, Thor does not have the foresight to prevent Loki from completing his ultimate act; taking Odin's place as king of Asgard. Thor knows full well the power his brother bestows and yet lets him loose when the Dark Elves attack. Sure, you could argue that he was desperate but he wouldn't have had to use Loki if he had dealt with his brother permanently after the actions of The Avengers. As it stands, Thor may have very well let his brother take all the power and perhaps the third film in phase 3 will deal with Thor picking up that mess.