'Removes enemy from sprite groups if 500 pixels left off the screen,
underneath the bottom of the screen, or right of the screen if shell'

def delete_if_off_screen(self, enemy):

    if (enemy.rect.x < (self.viewport.x - 300)):
        enemy.kill()
    elif (enemy.rect.y > self.viewport.bottom):
        enemy.kill()
    elif (enemy.state == c.SHELL_SLIDE):
        if (enemy.rect.x > (self.viewport.right + 500)):
            enemy.kill()
