Pygame Example Overriding Sprite kill method.

import pygame
from pygame.sprite import Group, Sprite

# Simple state
class State:
    def __init__(self, engine):
        self.engine = engine

    def on_draw(self, surface):
        pass

    def on_event(self, event):
        pass

    def on_update(self, delta, ticks):
        pass

# Simple display and state manager
class DisplayEngine:
    def __init__(self, caption, width, height, flags=0):
        pygame.display.set_caption(caption)
        self.surface = pygame.display.set_mode((width, height), flags)
        self.rect = self.surface.get_rect()
        self.clock = pygame.time.Clock()
        self.running = False
        self.delta = 0
        self.fps = 60

        self.state = State(self)
        self.on_quit = self.quit

    @property
    def width(self):
        return self.rect.width

    @property
    def height(self):
        return self.rect.height

    @property
    def size(self):
        return self.rect.size

    def main_loop(self):
        self.running = True
        while self.running:
            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    self.on_quit()
                else:
                    self.state.on_event(event)

            ticks = pygame.time.get_ticks()
            self.state.on_draw(self.surface)
            self.state.on_update(self.delta, ticks)
            pygame.display.flip()
            self.delta = self.clock.tick(self.fps)

    def quit(self):
        self.running = False

class Hitbox(Sprite):
    group = Group()

    @classmethod
    def create_image(cls, color, width, height):
        cls.image = pygame.Surface((width, height), pygame.SRCALPHA)
        cls.image.fill((0, 0, 0, 0))
        pygame.draw.rect(cls.image, color, (0, 0, width, height), 1)

    def __init__(self):
        Sprite.__init__(self)
        self.image = Hitbox.image
        self.rect = self.image.get_rect()

        Hitbox.group.add(self)

class Enemy(Sprite):
    group = Group()

    @classmethod
    def create_image(cls, color, width, height):
        cls.image = pygame.Surface((width, height))
        cls.image.fill(color)

    def __init__(self, position ,anchor="topleft"):
        Sprite.__init__(self)
        self.image = Enemy.image
        self.rect = self.image.get_rect()
        setattr(self.rect, anchor, position)
        self.center = pygame.Vector2(self.rect.center)
        self.velocity = 0.1
        setattr(self.rect, anchor, position)
        self.hitbox = Hitbox()

        Enemy.group.add(self)

    def kill(self):
        self.hitbox.kill()
        Sprite.kill(self)

    def update(self, delta):
        self.center.x -= self.velocity * delta
        self.rect.center = self.center
        self.hitbox.rect.center = self.center

        if self.rect.right < 0:
            self.kill()

class Label:
    def __init__(self, pen, text, position, anchor="topleft"):
        self.pen = pen
        self.image = pen.write(text)
        self.rect = self.image.get_rect()
        setattr(self.rect, anchor, position)
        self.anchor = anchor
        self.position = position

    def draw(self, surface):
        surface.blit(self.image, self.rect)

    def update(self, text):
        self.image = self.pen.write(text)
        self.rect = self.image.get_rect()
        setattr(self.rect, self.anchor, self.position)

class Pen:
    def __init__(self, font, color):
        self.font = font
        self.color = color

    def write(self, text, alias=1):
        return self.font.render(text, alias, self.color)

class ExampleState(State):
    def __init__(self, engine):
        State.__init__(self, engine)
        self.draw_hitboxes = True
        position = engine.rect.right - 10, 10
        pen = Pen(pygame.font.Font(None, 28), pygame.Color("lawngreen"))
        self.label = Label(pen, "Enemy Count: 0", position, "topright")
        self.enemy_count = 0

    def add_enemy(self):
        Enemy(self.engine.rect.center, "center")

    def on_draw(self, surface):
        surface.fill(pygame.Color('black'))
        Enemy.group.draw(surface)
        if self.draw_hitboxes:
            Hitbox.group.draw(surface)

        self.label.draw(surface)

    def on_event(self, event):
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_SPACE:
                self.draw_hitboxes = not self.draw_hitboxes
            elif event.key == pygame.K_e:
                self.add_enemy()

    def on_update(self, delta, ticks):
        Enemy.group.update(delta)
        if self.enemy_count != len(Enemy.group):
            self.enemy_count = len(Enemy.group)
            self.update_label()

    def update_label(self):
        text = "Enemy Count: {}".format(len(Enemy.group))
        self.label.update(text)

def main():
    pygame.init()
    engine = DisplayEngine("Example", 800, 600)
    Hitbox.create_image(pygame.Color("snow"), 50, 70)
    Enemy.create_image(pygame.Color("dodgerblue"), 200, 200)
    example = ExampleState(engine)
    engine.state = example
    engine.main_loop()

main()