We talk about fear as if it's always the same, but Divergent makes a good case that there are different types of fear: there's dread, panic, terror. (Yay for thesauruses.) In this, part of Tris's fear is wrapped up with something that she wants but doesn't quite trust. Tris doesn't have the same fear when it comes to jumping off buildings, for instance.

As Tris (and Four) make clear, Tris's fear landscape has a lot of powerlessness in various forms (birds, drowning, ocean waves, etc.), because Tris's big fear is losing control. But since people can control their fear landscapes, Tris can summon a symbol of power to combat the symbol of powerlessness: gun vs. birds. Although if she can control the fear landscape, why not get lots of guns, Matrix-style?

Tris at one point looks down on Al for not acting—for not making a choice (10.78). But here Tris makes a very heroic, self-sacrificing choice not to act, herself. Instead of shooting her family (in this fear simulation), Tris makes the choice to give her own life. And we get that choice very directly in Tris's italicized wish that she should be shot instead. It may look passive, but it's actually a pretty strong choice on her part.