A recurring theme in the Avengers films is Bruce Banner's horror at the carnage that ensues whenever he transforms into the Hulk. For example, in Age of Ultron
- As the Avengers are flying home after their initial mission, he is visibly traumatized by the knowledge that the Hulk has just killed or maimed large numbers of Hydra soldiers. Witness his reaction as Thor casually quips about all the enemies that the Hulk has just ploughed through.
- During Stark's party, Romanoff refers to him as "the guy who avoids fighting because he knows he will win".
- At the farmhouse, Banner rejects Romanoff's advances on the grounds that it's not safe for anybody to live with him, being as he is at risk of turning into the Hulk at the drop of a hat.
Add to this that previous films reference Banner's suicide attempt, which was prompted exclusively by his desire to put an end to the threat to others that the Hulk represents.
But then, why does he keep on going to missions with the Avengers? Clearly, when the Avengers go on a field mission, they don't want Banner's intellect, they want the Hulk's strength and invulnerability. Banner must be aware that accompanying the Avengers on a field mission carries a high probability of him turning into the Hulk and causing death and suffering to fellow human beings. Yet, he never says "I've had enough, I'm going to sit this one out, you guys are on your own", or anything to that effect. Why?