Annals I.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 dispatched 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."

Tiberius becomes Emperor and immediately executes Postumus. Well, actually Postumus is executed, but we don’t know by whom. Here is another point of dispute among historians. With this one, I actually agree with Tacitus’ implication that Tiberius was the instigator. Yet, a number of modern historians, despite Tiberius having a clear motive and ability to remove competition for the throne, think Tiberius is innocent.

The historians note that Tiberius ordered an inquiry into the murder of Postumus. A guilty man, they theorize, would not do such a thing. On top of this, Sallustius Crispus became panicked by the thought of inquiry and told Livia, who told Tiberius, to drop it. The inquiry was dropped, but not for fear of Tiberius being discovered, but for fear that he would look weak by deferring power to the Senate. Modern historians also point out that Tacitus is careful not to explicitly blame Tiberius. He simply claims the new reign kills him.

I would say that Tiberius is the only logical culprit. Augustus, who Tiberius claims made the order, had no reason to wait to kill Postumus. He had chosen exile for Postumus and had stuck to it for five years. I also don’t think Livia or anyone else had the power to order or achieve such a murder. Postumus was under heavy guards and only someone who controlled the guards could get to him.

As for the modern historians’ claims, Tiberius only ordered an inquiry. It never actually went through. It may have easily been a bluff that was never called. As for Tacitus being unsure, it may be that Tacitus wants everyone to look guilty. He’s spreading around blame to make Tiberius, Livia, Crispus and the whole administration look guilty, murderous and secretive.