Why did Nero order many of the Pisonian conspirators to commit suicide instead of having them executed by the sword?

No contemporary author offered a reason why Nero would have ordered that some of the conspirators should be "executed by suicide". However, the Pisonian conspirators were by no means the only high-ranking Romans that he ordered to commit suicide. Another high-profile example that springs to mind is the general, Gnaeus Domitius Corbulo.

One of the things that set the Pisonian conspiracy apart from the plots and conspiracies that preceded it in the reigns of earlier emperors was its scale.

Our most complete account of the conspiracy comes from Tacitus (Annals, Book XV). We know that there were at least 41 people involved in the Pisonian Conspiracy. There may well have been many others, but according to Cassius Dio, Tigellinus allowed others to go free in return for substantial bribes:

... many, as I have stated, were put to death, and many others, purchasing their lives from Tigellinus for a great price, were released.

• [Cassius Dio]

Of those conspirators we know, it is not always easy to determine their rank. It seems that 20 were Senators, 9 were Equites, 8 were soldiers, and 4 were women (including the freedwoman Epicharis who managed to commit suicide while being held in captivity and tortured). The conspiracy had succeeded in bringing together representatives of the Senatorial class, the Equites, and the military.

For convenience, I've listed the conspirators below, grouped according to their fate:

Executed or forced to commit suicide (19):

• Gaius Calpurnius Piso (S),
• Plautius Lateranus (S),
• Marcus Annaeus Lucanus ("Lucan") (S),
• Afranius Quintianus (S),
• Flavius Scaevinus (S),
• Seneca the Younger (S),
• Marcus Vestinus Atticus(S),
• Claudius Senecio (E),
• Vulcatius Araricus (E),
• Julius Augurinus (E),
• Munatius Gratus (E),
• Marcius Festus (E),
• Faenius Rufus (M),
• Subrius Flavus (M),
• Sulpicius Asper (M),
• Maximus Scaurus (M),
• Venetus Paulus (M),
• Antonia (W),
• Epicharis (F).

Exiled or denigrated (17):

• Novius Priscus (S),
• Annius Pollio (S),
• Glitius Gallus (S),
• Rufrius Crispinus (S),
• Cluvidienus Quietus (S),
• Julius Agrippa (S),
• Blitius Catulinus (S),
• Petronius Pricus (S),
• Julius Altinus (S),
• Caesennius Maximus (S),
• Cornelius Martialis (S),
• Flavius Nepos (S),
• Statius Domitius (S),
• Verginius Flavus (E),
• Musonius Rufus (E),
• Caedicia (W),
• Pompeius (M).

Pardoned or acquitted (5):

• Antonius Natalis (E),
• Cervarius Proculus (E),
• Statius Proxumus (M),
• Gavius Silvanus (M), (Subsequently committed suicide)
• Acilia (W).

E - Equites; S - Senator; M - Soldiers (miles); W - Women of Senatorial rank; F - Freedwoman;

Even if some of these were just Nero taking the opportunity of settling scores (as is suggested by Tacitus), this was a large-scale conspiracy that included popular Senators, senior military figures and individuals with privileged access to the emperor's person. It has rightly been described as "The beginning of the end for Emperor Nero" (albeit with the benefit of 20/20 hindsight).

The very existence of the conspiracy weakened the emperor's position. What Nero could not afford was a series of public trials that could inspire further opposition.

It has to be said that we do not know exactly how many of the 19 who paid the ultimate price in the aftermath of the conspiracy were actually ordered to commit suicide. In the case of Piso himself, we do know that he was ordered to commit suicide by Nero. As Vasily Rudich puts it:

Gaius Piso died in the customary manner - by severing his own veins. No less a custom by this time, and one he followed obediently - "for his love of his wife", we are told - was posthumous flattery of the emperor by the condemned in a testamentary document with the aim of securing the physical safety of the surviving members of the family, and even some piece of their inheritance, if by chance the entire estate was not to be confiscated.

• [Rudich, 2005, p106]

For many of the other conspirators, Tacitus is less clear. He states that Subrius Flavus, Sulpicius Asper, and the Centurions were executed according to military discipline. We are given some detail regarding the suicides of Seneca and Vestinus. Lucan's suicide is implied, as are those of Senecio, Quintianus and Scaevinus. For the remainder, he tells us only that:

the rest of the conspirators, met their end, doing and saying nothing that calls for remembrance.

• [Tacitus, Annals, XV, 70]

A further important factor that may have influenced Nero's decision is suggested the statement in Tacitus that he was worried about the negative effect of the publicity surrounding the executions. When Seneca's wife Pompeia Paullina attempted to emulate her husband's suicide, she was prevented from doing so on Nero's orders because he:

had no private animosity against Paulina, and did not wish to increase the odium of his cruelty

• [Tacitus, Annals, XV, 64]

So, why would Nero order some executions by suicide?

From Nero's viewpoint, the suicides offered several benefits:

1. The posthumous flattery from the victim (referred to above) could be used as propaganda to deflect people from the fact of the conspiracy (at the very least, it would feed Nero's ego!).
2. The fact of the suicide could be used as evidence of guilt, and so he might hope that it wouldn't further "increase the odium of his cruelty".
3. There was no need for a public trial which might inspire further opposition.
4. The scale of the conspiracy could be concealed.
5. The conspirators still ended up dead.

Sources

• Barrett, Anthony, Fantham, Elaine, & Yardley, John: The Emperor Nero: A Guide to the Ancient Sources, Princeton, 2016
• Rudich, Vasily: Political Dissidence Under Nero: The Price of Dissimulation, Routledge, 2005