I was about to make the argument that if batman killed the joker the only consequence for him would be his compromised morals, but if someone else killed the joker while he was in custody then they would at least lose their job and most likely go to jail and that's not comparable. Then I remembered that if a cop killed him they'd just get paid leave before they were acquitted of the murder and worst case would have to get a job in a different city. So yeah ACAB.