The Latin word for what we would usually call mercy, clementia, has caused some dispute among scholars. Some translate it as "clemency", which is a rather specific kind of mercy – that by superiors to inferiors.
Seneca is one of the major Stoic philosophers still read today and, among other strange and exciting endeavours in the course of a rather rock‘n’roll life, was tutor to a teenage Emperor Nero. A less enviable job would be difficult to imagine, apart, perhaps from that of Henry VIII’s Groom of the Stool, who assisted that expansive King in his ablutions.
Nero, not known for clemency or evenness of temper any more than Henry, later ordered poor Seneca to take his own life, which he duly did.
On Mercy is one of Seneca's Moral Essays, and was an address to Nero in the earliest years of his reign, when he apparently looked more promising and less of a maniac. Perhaps Seneca was squinting a little; Nero was already becoming established as cruel and tyrannical when it was written.
Regardless, On Mercy is the first known instance of what we now consider the "mirror for princes" genre – essentially literary outlining principles of conduct and structures of power for rulers. Of course, even that is in dispute – some attribute the roots of that genre to the ancient Greek historian Xenophon. Whoever got there first, it seems fair to allow that Nero needed a touch of guidance more than your average tyrannical despot. Among the virtues of a good ruler articulated by Seneca in the essay (no doubt to give the morally errant Nero a bit of a nudge) is clemency. A Roman leader sparing a conquered people rather than slaughtering them might have been considered virtuous in Seneca's time.
In the modern era, we wouldn’t consider any response to dominating those “outside” our physical or ideological borders appropriate, as the domination itself has thankfully mostly grown out of fashion. Though there is a collective longing for virtuous leaders, the sphere in which most of us exercise mercy has narrowed to within the parameters of individual interaction and is probably better thought of as compassion.
The Stoics, though enjoying something of a resurgence these days, are much misunderstood. They are seen as unfeeling, taciturn, above it all – that weird student at a 21st birthday party not talking to anybody but reading Nietzsche and eating raw kale from the bag because it “keeps things regular”. Stoicism is ultimately a means of living a good life by taking personal responsibility for our own actions, thoughts and feelings, and seeking to improve the world moment by moment starting with ourselves.
As a younger person, I was drawn to Stoicism, but initially misunderstood it, and became, for a time, a female iteration of that young man at the party. Only instead of Nietzsche, I was reading Seneca or Marcus Aurelius, and instead of kale I was eating Haribo (Stoic discipline doesn't come overnight), and instead of being at a party I was in my room alone.
I arrived at Trinity College at the tail-end of a dysfunctional childhood and worked 12-hour shifts in a dry cleaners to make rent. I had to miss lectures to work. Sometimes I was hungry, and couldn't help resenting the luckier, wealthier kids who took holidays and didn't have to panic when their ancient laptop broke. I was studying to get out of one sort of life and climb up into another. Most of my peers could not relate to that.
I viewed the world as systematically stacked against people like me, and in some ways it can be. But the Stoics taught me to take responsibility and have compassion for others rather than pity for myself. Sadly, equal opportunity is not something we are born into, but we can push forward with a Stoic outlook. The narrative of systemic victimisation can make us feel trapped inside the circumstances of our birth, but with a compassionate outlook and a Stoic sense of ourselves as works in progress, we can improve our lives. There’s no use waiting for Nero to have mercy and do it for us.