TACITUS, Annales I.6
[I.vi] Primum facinus novi principatus fuit Postumi Agrippae caedes, quem ignarum inerumumque quamvis firmatus animo centurio aegre confecit. nihil de ea re Tiberius apud senatum disseruit: patris iussa simulabat, quibus praescripsisset tribuno custodiae adposito, ne cunctaretur Agrippam morte adficere, quandoque ipse supremum diem explevisset. multa sine dubio saevaque Augustus de moribus adulescentis questus, ut exilium eius senatus consulto sanciretur perfecerat: ceterum in nullius umquam suorum necem duravit, neque mortem nepoti pro securitate privigni inlatam credibile erat. propius vero Tiberium ac Liviam, illum metu, hanc novercalibus odiis, suspecti et invisi iuvenis caedem festinavisse. nuntianti centurioni, ut mos militiae, factum esse quod imperasset, neque imperasse sese et rationem facti reddendam apud senatum respondit. quod postquam Sallustius Crispus particeps secretorum (is ad tribunum miserat codicillos) comperit, metuens ne reus subderetur, iuxta periculoso ficta seu vera promeret, monuit Liviam ne arcana domus, ne consilia amicorum, ministeria militum vulgarentur, neve Tiberius vim principatus resolveret cunta ad senatum vocando: eam condicionem esse imperandi, ut non aliter ratio constet quam si uni reddatur.
[1.6] The first crime of the new reign was the murder of Postumus Agrippa. Though he was surprised and unarmed, a centurion of the firmest resolution despatched him with difficulty. Tiberius gave no explanation of the matter to the Senate; he pretended that there were directions from his father ordering the tribune in charge of the prisoner not to delay the slaughter of Agrippa, whenever he should himself have breathed his last. Beyond a doubt, Augustus had often complained of the young man's character, and had thus succeeded in obtaining the sanction of a decree of the Senate for his banishment. But he never was hard-hearted enough to destroy any of his kinsfolk, nor was it credible that death was to be the sentence of the grandson in order that the stepson might feel secure. It was more probable that Tiberius and Livia, the one from fear, the other from a stepmother's enmity, hurried on the destruction of a youth whom they suspected and hated. When the centurion reported, according to military custom, that he had executed the command, Tiberius replied that he had not given the command, and that the act must be justified to the Senate. As soon as Sallustius Crispus who shared the secret (he had, in fact, sent the written order to the tribune) knew this, fearing that the charge would be shifted on himself, and that his peril would be the same whether he uttered fiction or truth, he advised Livia not to divulge the secrets of her house or the counsels of friends, or any services performed by the soldiers, nor to let Tiberius weaken the strength of imperial power by referring everything to the Senate, for "the condition," he said, "of holding empire is that an account cannot be balanced unless it be rendered to one person."