Short sighted history always bothers me. Case in point:
The phrase is common enough in Ciceronian rhetoric when talking about external enemies, but he also makes very very clear the dangers of its domestic application.
O, how I wish this case afforded me the opportunity and the ability to proclaim that Lucius Saturninus, enemy of the Roman people, was killed by Gaius Rabirius.—Your shouting does not disturb me at all. Rather, it reassures me since it shows that there are some foolish citizens but not many. Never would the Roman people who remain silent have made me consul if they thought I would be shaken by your shouting. How much quieter your outcries have become already! Yes, you are checking your voice, informer upon your stupidity, witness to your paltry numbers!
Gladly, as I say, would I acknowledge, if I were in truth able or even if I were at liberty to do so, that Lucius Saturninus was killed by the hand of Gaius Rabirius. I would deem it a most glorious misdeed. But seeing that I cannot do this, what I will confess will be less efficacious for his reputation but not less for the charge against him. I confess that Gaius Rabirius took up weapons for the purpose of killing Saturninus. How is that, Labienus? What fuller confession, what more serious charge against my client were you expecting? Unless, of course, you do reckon that there is a difference between a man who has killed a man and a man who was armed for the purpose of killing a man. If it was wrong for Saturninus to be killed, weapons cannot be taken up against Saturninus without entailing a crime. If, however, you concede that weapons were taken up lawfully, then, by necessity, you must concede that he was killed lawfully.
This is Cic. Rab. 18ff. In short, being declared an enemy of the people meant it was legitimate for anyone to kill you. There are of course many many more examples from the Catilinarian conspiracy and Cicero’s attacks on Antony.