"I Heard a Fly Buzz When I Died" isn’t a normal nature poem in any way. No fluffy clouds or bubbling streams here. But Dickinson does spend a lot of time on that fly. In a lot of ways, it’s the main symbol and one of the main characters in the poem. So in that sense, this is a poem about how humans relate to the natural world.
The fly is a symbol of evil and chaos. It is one half of a subtle religious allegory in this poem, a struggle between the forces of darkness and light.