That quote is an excerpt from one of his letters.
https://trove.nla.gov.au/newspaper/article/93970818
Here is a larger quotation:
Solitary amidst men I return to my room to think alone and to abandon myself to my melancholy. Where does it lead me to-day? Towards death. In the springtime of life, I may hope to live long. I have been absent from my country six or seven years. What pleasure shall I have in four months in seeing once more my companions and parents? Can I not conclude, from the sweet sensations awakened by the remembrance of my childhood, that may happiness will be complete? What mad fellow urges me on to self-destruction? But what can be done in this world? Since I have to, is it not better to kill myself? If I was over sixty years of age I would respect the prejudices of my fellow-creatures, and would wait patiently for nature to accomplish its work; but since I commence to experience unhappiness, and since nothing gives me pleasure, why should I endure days in which I succeed in nothing? How far men arc from nature! How cowardly, vile, and cringing they are! What is the spectacle I shall witness in mv country? Fellow-countrymen loaded with chains, all trembling, kissing the hand that oppresses them. They are no longer brave Corsicans animated by heroic virtues, and the enemies of tyrants, luxury, and base courtesans. Proud and full of the noble consciousness of his own personal importance, a Corsican was formerly happy. If he had I occupied his day in transacting public business, the night was spent in the tender arms of & dearly-loved wife; his reason and his enthusiasm obliterated all the troubles of the day, tenderness and nature rendered the night comparable to that of gods. But with liberty those happy days have vanished like a dream. Frenchmen ! Not content with having despoiled us of all we loved, you have corrupted our manners. The present condition of my country, and my powerlessness to change it, are additional reasons for me to leave* a land where I am obliged by duty to praise men whom I ought by virtue to hate. When I arrive in my country how am I to act, and what am I to do? When the mother country has ceased to exist, a good citizen should die. If I had to destroy but one man in order to deliver my fellow-countrymen I would start at once. I would plunge the avenging dagger up to the hilt in the breast of the tyrant. My life is a burden, because I taste no pleasure, and because, for me, everything is wearisome. My life is a burden, because I live, and must probably always live, with men whose thoughts and manners are as different from mine as the silver moonlight is different from the light of the sun. I cannot, then, follow the only manner of living that could make life bearable for me, whence it follows that I feel aversion for everything.