harold left after having an attack of conscience over what he is doing to paul - lou finds out or thinks that he knows what harold is doing so throws the brick through the window to take suspicion away from harold as he is not around so cannot be blamed this is the only incident that lou does but when harold returns he admits to paul what he and lou on the one occasion did.
i am not sure which episode the police confirmed that it was tony but i will try to look through the episodes to find out