The first exploit of the new reign was the murder of Agrippa Posthumus. A centurion of undaunted resolution attacked him by surprise. Though unprovided with arms, the young man did not easily yield: he fell after a stout resistance. Of this event Tiberius made no report to the senate,content with hinting a pretended order of his deceased father, by which the centurion, charged with the custody of Agrippa's person, was commanded to dispatch him, as soon as the emperor breathed his last. Augustus, it is true, had arraigned the character and conduct of the young man in terms of asperity; he had even banished him by a decree of the senate, but it is equally true, that he never imbrued his hands in the blood of his kindred; nor is it probable that, for the security of a stepson, he would have doomed to death a descendant from himself. The stronger presumption is, that Tiberius and Livia, the former impelled by his dread of a rival, and the latter by the malice of a step-mother, were accomplices in the murder. When the assassin, in the military phrase, reported to Tiberius, that what he had given in orders was duly executed, the reply of the new emperor was, that he had given no such orders, and for what was done the centurion must answer before the senate.

A disavowal so very extraordinary gave the alarm to Sallustius Crispus, a minister then in favour, and trusted with the secrets of the court. The warrant for the execution had passed through his hands. He dreaded a public examination; well aware that, whether he disclosed the truth, or attempted to disguise it, his own danger would, in either case, be precisely the same. To ward off the blow, he remonstrated to Livia, that the secret counsels of the imperial family, the conducts of ministers, and the actions of the centurions, ought to be veiled from the public eye. By referring too much to the senate, the prince would weaken his own authority: that men should be accountable to the sovereign only, was a branch of the imperial prerogative; and if Tiberius departed from it, he ceased to reign.