 The Avengers become divided , both over how to approach Loki and the revelation that S.H.I.E.L.D. plans to harness the Tesseract to develop weapons as a deterrent against hostile extraterrestrials . As the group argues , Barton and Loki 's other possessed agents attack the Helicarrier , disabling its engines in flight and causing Banner to transform into the Hulk . Stark and Rogers try to restart the damaged engine , and Thor attempts to stop the Hulk 's rampage . Romanoff fights Barton , and knocks him unconscious , breaking Loki 's mind control . Loki escapes after killing Coulson and ejecting Thor from the airship , while the Hulk falls to the ground after attacking a S.H.I.E.L.D. fighter jet . Fury uses Coulson 's death to motivate the Avengers into working as a team . Stark and Rogers realize that for Loki , simply defeating them will not be enough ; he needs to overpower them publicly to validate himself as ruler of Earth . Loki uses the Tesseract , in conjunction with a device Selvig built , to open a wormhole above Stark Tower to the Chitauri fleet in space , launching his invasion . 
