I've been reading a book by James Romm entitled Dying Every Day: Seneca at the Court of Nero, and am more than ever fascinated and disturbed by the life of this very able proponent of Stoicism and very able man, who nonetheless taught and facilitated the reign of a possibly demented and undoubtedly unpredictable tyrant.
While I have issues with the sometimes contrived, sometimes cloying and sometimes pompous style of his philosophical writings, there can be no question that he must be considered one of the great Roman Stoics, and that his thought and works have been immensely influential throughout the centuries from his death (he's also a significant literary figure). He's thought to have championed a milder version of Stoicism which appealed to the Church Fathers (Tertullian called him "our Seneca"). Unlike Epictetus who did not write, and Marcus Aurelius who wrote only for himself, Seneca wrote understanding his work would be published (and no doubt seeing to it himself, as was not unusual for the time), and possibly even with posterity in mind. So it can be assumed that he did so carefully and in most cases for a purpose.
In some cases he clearly wrote with Nero in mind, such as in his work On Anger, and in some cases with his own advantage in mind, as in the dreadful Consolation to Polybius. In some cases, in his letters, he may have done so not for any personal or political purposes but with philosophy in mind, or at least the good life. It's difficult to ascribe purpose to his work in other cases, where he seems--perhaps deliberately--ambiguous.
"Cautious" may be a good word to describe him after his return to Rome from exile in Corsica at the behest of Nero's mother, the formidable Agrippina. He had managed to run afoul of the princeps in the past and would not want to do so again. It's understandable that he would strive not to provoke imperial anger. It would also seem he was ambitious. This ambition must, I think, be considered in the context of the time. Romans of his class (though he was a provincial) were supposed to be ambitious, to attain wealth and high office, to be renowned.
But it's hard to reconcile ambition and wealth with Stoic tenets, and Seneca became very wealthy and very powerful. Pursuing public service, though, is consistent with Stoicism.
When brought back from Corsica to tutor the young Nero, then, it's possible, perhaps even probable, that Seneca thought he was being given a chance to do good; to mold the future ruler of the known world to be wise and merciful (as described in his work On Mercy). And he may have thought that becoming wealthy and powerful while doing so appropriate and even necessary to the task. How else influence an emperor, who would likely only listen to the wealthy and powerful? But it's difficult to accept his reply to those of his time (and subsequent times) who condemned him for his acquisition of wealth as being unworthy of a philosopher, particularly a Stoic, except with a scowl or grimace. He noted that disdain for wealth was to be expected from a Stoic Sage, but he had not yet attained that status--he was still trying to be a Stoic Sage. When he became one, presumably, he would not want to be wealthy any longer. An unsatisfying reply, I think.
But what of his conduct as Nero "matured" in tyranny? Romm notes Seneca may have played a part in freeing Nero from the dominance of his mother. She was not herself a saintly figure by any means and may well have been thought to be a bad influence on her son, but Nero unbound was a dangerous man. It can't be said with certainty that Seneca was involved in Nero's matricide, but it seems clear that he wrote the speech given by Nero to the Senate in which his mother was reviled and her death justified. Seneca can be said to have done what he could to restrain Nero from his social excesses--his acting, singing, chariot racing and writing of poetry was scandalous given Roman tradition--but eventually he was engaged in this conduct openly. But just what he did otherwise to restrain Nero is unclear, if he did anything. Seneca wrote a great deal but said almost nothing at all regarding what took place while he was a senior consultant to the emperor.
Eventually, as Nero grew more dangerous, Seneca offered to grant him all his wealth and retire from politics--twice. It can be argued this was motivated by purely practical considerations, though, and in an effort at self-preservation. This is likely so. But it's difficult to condemn someone in Nero's court for being fearful. And Seneca could have felt fear not only for himself but for his brothers and his nephew, whom we know as Lucan. They were known to and within the reach of Nero.
It may be that Seneca believed he was caught in a trap at least in part of his own making, and he could not see his way out. But he wrote admiringly of those, including Cato, who took their own lives when life became intolerable and they could not live with honor and in virtue. Why didn't he take advantage of this option himself? Eventually he did, of course, but only when ordered to dispose of himself by the emperor, having been implicated in a plot to kill Nero. His nephew Lucan was involved in the plot, but Seneca declined to participate. Cowardice? Caution?
Not cowardice, I think, at least not in the sense of fear of suffering. His death took a long time. When cutting his veins would not work effectively, he took hemlock. All accounts are he died nobly though in great pain. So caution is more likely; caution and indecision, which are not unusual in old men, and he was old then and likely very tired.
It's bewildering that someone could write so well regarding virtue and the good and yet be so hesitant and ineffective in being virtuous and good in great things. But perhaps this is a case of the spirit being willing and the flesh weak. Perhaps it's unfair to think a man in his place to have acted in bad faith, dishonestly. It's possible to be a good but weak man. We should consider it possible that Seneca should be pitied rather than reviled.