Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
As regards the right of self-redress and in particular the avenging of blood, we still find perhaps in legends an echo of the original principle that a murderer, or any one who should illegally protect a murderer, might justifiably be slain by the kinsmen of the person murdered; but these very legends characterize this principle as objectionable,(3) and from their statements blood-revenge would appear to have been very early suppressed in Rome through the energetic assertion of the authority of the community.
3. The story of the death of king Tatius, as given by Plutarch (Rom. 23, 24), viz. that kinsmen of Tatius had killed envoys from Laurentum; that Tatius had refused the complaint of the kinsmen of the slain for redress; that they then put Tatius to death; that Romulus acquitted the murderers of Tatius, on the ground that murder had been expiated by murder; but that, in consequence of the penal judgments of the gods that simultaneously fell upon Rome and Laurentum, the perpetrators of both murders were in the sequel subjected to righteous punishment--this story looks quite like a historical version of the abolition of blood-revenge, just as the introduction of the -provocatio- lies at the foundation of the myth of the Horatii. The versions of the same story that occur elsewhere certainly present considerable variations, but they seem to be confused or dressed up.
In like manner we perceive in the earliest Roman law no trace of that influence which under the oldest Germanic institutions the comrades of the accused and the people present were entitled to exercise over the pronouncing of judgment; nor do we find in the former any evidence of the usage so frequent in the latter, by which the mere will and power to maintain a claim with arms in hand were treated as judicially necessary or at any rate admissible.
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Reference address : https://www.ellopos.net/elpenor/rome/1-11-law-justice.asp?pg=4