The key factor in the Avengers coming together as a team to defeat Loki and the Chitauri in their titular movie was the death of Phil Coulson. When the God of Mischief had escaped from his cage on the S.H.I.E.L.D. Helicarrier, Coulson threatened him with an experimental weapon that S.H.I.E.L.D. had crafted using the Asgardian Destroyer armour. However, Loki used his power of illusion to attack Coulson from behind and thrust the sharp end of his sceptre through the agent's chest. He died there and then, with his dying words suggesting that he knew his death would act as the catalyst for the Avengers working as a team. Of course, he would later be brought back to life to star in the Agents of S.H.I.E.L.D. television series, making the key scene in the movie seem almost pointless and having had everyone watching falsely believe that they'd never see Clark Gregg's character again.
