Seneca wrote two plays about the Stoic hero, Hercules. It’s sometimes claimed that his plays seem totally divorced from his philosophy and portray violent scenarios, with little philosophical content. However, these two plays, set just before the twelve labours began, and just after he completed the final one, both contain clearly philosophical remarks and focus on well-known Stoic themes. We find obvious references in both plays to the notion that the external consequences of actions are morally indifferent, only our intentions can make us virtuous or vicious. We also find a number of other philosophical remarks, quoted below.
The Madness of Hercules (Hercules Furens)
Hercules is driven temporarily insane by the goddess Hera (Juno) and kills his wife and children, an awful tragedy he must somehow learn to live with. A major Stoic theme in this play is therefore the notion that we cannot be blamed for the unintended consequences of our actions, only our intentions are morally relevant. We learn from Hercules that even the most tragic act must be forgiven if it’s been done by mistake. Hercules consulted the Oracle of Delphi to discover how he could atone for this atrocity and this led to him undertaking the famous twelve labours, spanning the next twelve years of his life.
Chorus: Known to but few is untroubled calm, and they, mindful of time’s swift flight, hold fast the days that never will return. While the fates permit, live happily; life speeds on with hurried step, and with winged days the wheel of the headlong year is turned. 
Megara: What the wretched overmuch desire, they easily believe. 
Megara: Who can be forced has not learned how to die. 
Amphitryon: … things ’twas hard to bear ’tis pleasant to recall. 
Amphityron: What man anywhere hath laid on error the name of guilt? 
Hercules on Oeta (Hercules Oetaeus)
This is the story of Hercules’ death. Having completed the twelve labours, and overthrown King Eurytus, he seeks to take the slave girl Iole as his wife. However, his existing wife, Deianira, becomes jealous and tricks him into wearing a cloak imbued with what she mistakenly believes is a love potion. It turns out she was herself tricked, and the potion contains the Hydra’s blood, which poisons Hercules and kills him. Again, this story touches on the Stoic theme that the consequences of our actions are morally indifferent, and that our intentions alone determine our moral character. In this instance, it’s Deianira, though, who’s actions result in an unintentional catastrophe.
Chorus: Happy is he whoever knows how to bear the estate of slave or king and can match his countenance with either lot. For he who bears his ills with even soul has robbed misfortune of its strength and heaviness. 
Deianira: He has scorned all men, who first has scorn of death; ’tis sweet to go against the sword.
Chorus: Whoever has left the middle course fares never in path secure. […]To our undoing, high fortunes are by ruin balanced. 
Hyllus: Why dost drag down a house already shaken? From error spring wholly whatever crime is here. He does no sin who sins without intent. 
Hyllus: Life has been granted many whose guilt lay in wrong judgement, not in act. Who blames his own destiny? 
Hyllus: But Hercules himself slew Megara, pierced by his arrows, and his own sons as well, shooting Lernaean shafts with furious hand; still, though thrice murderer, he forgave himself, but not his madness. At the source of Cinyps ‘neath Libyan skies he washed away his guilt and cleansed his hands. 
Deianira: […] sometimes death is a punishment, but often ’tis a boon, and to many a way of pardon has it proved. 
Hylus: Give o’er now, mother, I beseech thee, pardon thy fate; an error is not counted as a crime. 
Hercules: Whate’er in me was mortal and of thee, the vanquished flame has borne away my father’s part to heaven, thy part to the flames has been consigned. […] Let tears for the inglorious flow; valour fares starward, fear, to the realms of death. 
At the conclusion, it’s explained that Hercules bore his death with a countenance “such as none e’er bore his life”, and that “joyous did he mount his funeral pyre”, with indifference to the flames. Like a Stoic then: “How calmly he bore his fate!”