To be fair, there is a lot of suicidal ideation and depression going on but it's entirely reframed as an exalted state of enlightenment. This state of absolute despair is perceived as the step before martyrdom. It's not a prelude to a weak downfall towards non-existence but to a courageous ascension towards the sublime. It turns a painful psychological state into a beautiful sacrifice. It takes the desire to cease existing and makes it feel like the highest form of enlightenment.
This is how you can summarize his philosophy:
"As long as liberty triumphs, what does it matter if my name is tarnished or if I perish?"
He knows true heroes perish and are slandered, so he expects it (that's in the introduction to the Institutions républicaines), but all that matters to him is that he offered his life as a monument to the truth and that his name and example will live on as a myth regardless. That he's transcending humanity into becoming a myth:
"I despise the dust of which I am composed and that speaks to you; one may persecute it and make this dust die! But I defy anyone to rip from me this independent life I have given myself in the centuries and in the skies."
People might see a contradiction here: how can he live forever if he's dead? But to him it's totally logical. If he cannot live free, then he will not live at all, and death becomes his true form of existence. There is no contradiction to him because he separates existence into two distinct categories: the dust (the physical, biological body that can be chained, starved, or guillotined) and the independent life (the moral legacy, the virtue, the idea).
For Saint-Just, the "dust" is merely a temporary vessel. If that vessel can no longer exist in absolute liberty, then keeping it alive through compromise or submission is the true death. To live on one's knees is to reduce oneself to nothing but dust. It's what he explains in the two paragraphs that precede the famous "centuries and skies" quote:
"Circumstances are only difficult for those who recoil before the tomb. I implore it, the tomb, as a blessing of Providence, so that I may no longer be a witness to the impunity of crimes plotted against my motherland and humanity.
Certainly, it is leaving very little behind to quit a miserable life, in which one is condemned to vegetate as either the accomplice or the impotent witness of crime…"
Therefore, letting the dust die is not an act of destruction; it is an act of liberation. It is the moment the idea is permanently freed from the fragile, corruptible flesh. This is exactly what drives his final weeks. He realizes that a living man can be compromised, bribed, worn down, or defeated by politics. A living man has to deal with the messy, frustrating friction of human nature. To be truly Incorruptible is to die for the Ideal.
Because Saint-Just famously suppressed his emotions by hiding behind an uncompromising, inflexible mask of ice and marble, people assume he lacked human feeling. That he was some kind of "sociopath". In truth, it was a philosophical exercise, the highest form of performance where your life becomes a role of absolute sincerity. There is no cynicism here. Only pure belief.
Beneath the facade he is forcing onto himself, there was a deeply sensitive (in this sense), hyper-idealistic, and profoundly exhausted young man dealing with what we would now classify as massive psychological pressure, depression, and burnout. But his century elevated this state not as a dysfunction but as perfection. His stoicism wasn't a lack of emotion; it was an intense, violent containment of emotion. He used an overwhelming amount of passion to act that "cold". Most people don't see the volcano underneath the ice; they only see the frost. But people from his time saw it, hence Barère’s quote: "He has a mind of fire and a heart of ice."
However, contemporary society - the one that rose from Thermidorian cynicism and bourgeois liberalism - is profoundly uncomfortable with the concept of the absolute. It teaches the value of self-interest, compromise, moderation, and keeping our heads down (and on) to survive. On the other hand, Saint-Just represents the terrifying, intoxicating (and therefore sublime) concept of total commitment and devotion to an Ideal. And that Ideal is Liberty and Independence. In bourgeois liberalism, liberty is a legal fiction and theory; for someone like Saint-Just, it's a sacred, transcendental force.
"Those who make revolutions by halves do nothing but dig their own graves."
He is rejecting the very concept of middle ground not just in the realm of politics but in life itself. To the contemporary audience, that level of conviction is terrifying. It cannot be compromised with or tempted with wealth, glory or power. It is much safer and easier for a contemporary mind to label that "fanaticism" and "madness" or to believe he was bluffing than to sit with the uncomfortable truth that a human being was capable of loving an ideal - in this case liberty - so much that he viewed his own life, and the lives of others, as entirely secondary to it:
The day I become convinced that it is impossible to give the French people customs that are gentle, energetic, and sensitive, I shall stab myself.
It's a secular form of martyrdom that cannot be comprehended in our era. If he's not dying for a God, how can he be dying for Liberty? Because Liberty was their secular goddess.