why did the young soldier feel like it was his fault that the mortar fire targeted them

The young soldier wanted to show KIowa his girlfriend's picture, so he pulled in out in the middle of the night and shone a flashlight on the photo. Almost the second he turned on the flashlight, mortar began exploding around them. Kiowa was killed.

The boy wanted to confess. He wanted to tell the lieutenant how in the middle of the night he had pulled out Billie's picture and passed it over to Kiowa and then switched on the flashlight, and how Kiowa had whispered, "Hey, she's cute," and how for a second the flashlight had made Billie's face sparkle, and how right then the field had exploded all around them. The flashlight had done it. Like a target shining in the dark.