Mukuro usually works alone when he kills other player so number-wise he on moral high-ground. 

And it my personal believe that if they didn't want pker jumping other out of no where to would have probably made it so skills that require you to sneak up someone are less effective on other players. Otherwise telling those who use assassin specs not to jump people is like the police handing a dangerous lunatic a assault rifle and a bomb and telling him not do terrorize the city with it, it just seem like self-contradiction.

Plus the last Black King supposedly worst then Mukuro for basically making night time in the servers a Living Hell for player wondering about.

Though understandably Mukuro no saint within the game, sometimes he drag thing out for the other player who insulted him by knowing using a weak gun to harm their avatar when they no longer has a chance of winning,sometime he stun one who offended and push them off a roof so they would die for fall damage. Accossing to from the Admin and co said pain is still a feature in this game and dying would make your sore in the morning.