How does your submission match the theme?
The shield is both defensive to protect yourself from bullets, and offensive allowing you to defeat opponents. This also makes enemies dual natured, enabling you to deal more or less damage depending on who you leave alive.

Third-party resources
One piece of music by Tristan Lohengrin. Creative Commons Attribution type license.

Contributors
Chris Holmes - Programmer/Artist/Sound Effects/everything.
Tristan Lohengrin - one piece of music found online.