1. In principle: Because vigilante "justice" undermines the rule of law, even if the rulers are themselves acting unjustly. In some rare instances tyrannicide is justifiable but only as a last resort and when carried out not by any old Joe or Sally but by those who have at least some form of legitimate authority. When we start praising those who take the law into their own hands we start down the road to anarchy. Recall Thomas More's speech in A Man for All Seasons to his future son-in-law, Will Roper, about Roper being willing to cut down all the laws in England to go after More's servant who might have been informing on More--Roper said More would give the Devil the benefit of Law and More said, yes, indeed I would because the law is like trees standing as a windbreak against the gales of evil and if you chop down all the laws/trees you have nothing to appeal to when the wind turns on you.
2. In practice: Because it will turn a murderer into a martyr and obscure Tiller's own crimes; the cause of truth will be set back.