All bad precedents began as justifiable measures.Sounded too modern and too pat so I went looking for a source.
I think it is actually a paraphrase of a comment by Sallust in his The Conspiracy of Catiline. Bold highlight:
But who it may be asked, will blame any severity that shall be decreed against these parricides of their country? I answer that time, the course of events, and fortune, whose caprice governs nations, may blame it. Whatever shall fall on the traitors, will fall on them justly; but it is for you, Conscript Fathers, to consider well what you resolve to inflict on others. All precedents productive of evil effects, have had their origin from what was good; but when a government passes into the hands of the ignorant or unprincipled, any new example of severity, inflicted on deserving and suitable objects, is extended to those that are improper and undeserving of it. The Lacedaemonians, when they had conquered the Athenians, appointed thirty men to govern their state. These thirty began their administration by putting to death, even without a trial, all who were notoriously wicked, or publicly detestable; acts at which the people rejoiced, and extolled their justice. But afterward, when their lawless power gradually increased, they proceeded, at their pleasure, to kill the good and the bad indiscriminately, and to strike terror into all; and thus the state, overpowered and enslaved, paid a heavy penalty for its imprudent exultation.So it is misattributed but the translation is actually reasonably close.
Within our own memory, too, when the victorious Sylla ordered Damasippus, and others of similar character, who had risen by distressing their country, to be put to death, who did not commend the proceeding? All exclaimed that wicked and factious men, who had troubled the state with their seditious practices, had justly forfeited their lives. Yet this proceeding was the commencement of great bloodshed. For whenever anyone coveted the mansion or villa, or even the plate or apparel of another, he exerted his influence to have him numbered among the proscribed. Thus they, to whom the death of Damasippus had been a subject of joy, were soon after dragged to death themselves; nor was there any cessation of slaughter, until Sylla had glutted all his partisans with riches.