I've been reading everything I can about this remarkable figure of the early Roman Republic, Publius Valerius Publicola. Most of us know him from the fact that the authors of the Federalist Papers took his name as their pseudonym.
Though almost everything we know about his life is remarkable, of particular interest to me was the fact that he passed a law making it legal for any citizen of Rome to execute any man who dared to make himself a king, even without due process.
I know that our knowledge of early Roman law isn't as comprehensive as it should be (for instance, even the Twelve Tables, which came well after Publius' death, aren't extant). However, do any of you happen to know if that law was still on the books by the time Caesar was assassinated?
Did the assassins murder Caesar under color of law?
This was actually a huge debate in the immediate aftermath. Tyrannicide was legal, and it was the goal of the liberatores to have Caesar declared a tyrant, thus making their actions legal. Antony and the Caesarians, on the other hand, refused to accept this. A compromise was hashed out in Caesar was not declared a tyrant, but the liberatores were granted immunity from all prosecution. Then Antony invalidated this with his funeral speech and by reorganizing the Caesarian faction, which drove the liberatores from Rome.
The action itself was absolutely illegal, as shutupshake points out below. Rather or not it was justified would have depended on which Roman you happened to ask....
The assassins would have justified it by saying that Caesar was assuming powers that went against the traditions of the Republic, and that he had to be stopped. The actual, personal motivations of the various assassins is a matter of speculation. Surely some of them believed they were saving the Republic. Some may have been there simply because they disliked Caesar, or were jealous of his power . Caesar's supporters, in the aftermath of the killing, proscribed a large number of upper-class people, including some who had no known part in the conspiracy but were known to sympathize with the assassins. Among the victims of this proscription was Cicero, who wrote in an extant letter that he was sad the conspirators hadn't asked him to join in, and also taken care of Mark Anthony while they were at it. The motivation behind this proscription was multi-fold, but outside of revenge one motive was surely money... proscribed people forfeited their estates, and the new leadership (the "Second Triumvirate" of Caesar's grand-nephew Octavian, Mark Anthony, and Lepidus) needed money to prosecute a war against Brutus and Cassius, two of the main conspirators who were raising an army in Greece to march on Rome to (they claimed) restore the Republic. I mention this about the proscriptions because it was a sort of "punishment" for the assassination, although the true motive is dubious and was likely a mix of the money and some good old fashioned revenge, with the added bonus of getting rid of people who might be a problem down the line....
This was a messy, messy time in Roman history. Caesar's death created a giant power vacuum, and led to another decade and a half of bloody civil war as the Second Triumvirate defeated B and C, and then began fighting against each other for power. The eventual victor was Octavian, who became the first Roman Emperor (Augustus). It's worth noting that Plutarch and Suetonius, who are used as primary sources for this period, were writing long after the fact. Presumably they had access to now-lost period sources... but the veracity of these cannot be known because they are lost.
So, illegal: Yes. Justified: Depends on who you would have been asking...
I recommend Tom Holland's book "Rubicon: The Last Days of the Roman Republic" as a good popular history introduction to this era, if you haven't read it already (it's pretty well known I think). Other source: I was a classics major in undergraduate college, many a year ago. For the love of great caesar's ghost, if I've gotten any of this wrong, feel free to correct me professional historians....
EDIT: It's been suggested that "Rubicon" is not the best potential source here, as it is a pop history kind of text. Another potential source is Eric Gruen's "The Last Generation of the Roman Republic".