无法删除类的实例

我开始用python编写代码，然后使用pyzero制作一个简单的游戏。游戏结束后，我想删除某些类型类的所有现有实例，以允许游戏重新开始。我有该类所有实例的列表，但是使用remove（self）似乎会导致我无法解决的逻辑问题。

class Ball(Actor):
    ball_list = []

    def __init__(self, actor):               
        Actor.initiate_actor(self,"ball")
        Ball.ball_list.append(self)
        self.alive = True

    def kill(self):
        if self.alive:
            self.alive = False
            Ball.ball_list.remove(self) 


def new_game():
    global game_over, score

    for actor in Ball.ball_list:
        actor.kill()

    score = 0
    game_over = False


def draw():
    global game_over
    if game_over:
        screen.clear()
        screen.draw.text("Game Over", center = (WIDTH/2, HEIGHT/2), color = 'white')

    else:
        screen.clear()
        backdrop.draw()  
        for actor in Ball.ball_list:
            if actor.alive:
                actor.draw()