In what story of Régnier does a man commit suicide without understanding the reason?

Question

In the suicide note of Ryūnosuke Akutagawa, he talks about a story by Régnier:

No one has yet written candidly of the mental state of one who is to commit suicide. This may be due to the self-respect of the suicide victim, or perhaps a lack of psychological interest in his own state of mind. In this, my final letter to you, I will endeavour to make clear my mental state. But even if I should fail to convey my motives to you accurately, I will still be satisfied in the attempt. In one of his short stories, Régnier depicts a man who commits suicide, but does not himself understand for what reason. You can find many motivations for suicide in the police section of the newspaper, from fiscal hardship or ill health to mental anguish. In my experience these do not represent the totality of motives, but only speak of the journey towards the true reason. Those who commit suicide are for the most part as Régnier depicted, unaware of their real motivation. Suicide, as with all our actions, involves a complexity of motives. In my case, I am driven by, at the very least, a vague sense of unease: some indistinct sense of unease towards my future. It is possible that you will not be able to take me at my word. Ten years of experience have taught me that for those who are not my close and constant acquaintances, my words dissipate like a song in the wind. So I would not blame you…

Which Régnier and which story or text is Akutagawa referring to?

Answer 1

In his book Le suicide et la morale (1922), Albert Bayet wrote,

Henri de Régnier fait parler un homme qui se tue sans savoir pourquoi, sans crainte ni désespoir, sans chagrin de cœur, uniquement parce qu'il est « destiné » au suicide (3).

Translation:

Henri de Régnier lets a man speak who kills himself without knowing why, without fear or despair, without grief/chagrin, solely because he is "destined" for suicide (3).

The footnote (3) says,

H. de Régnier, L'inexplicable dans Couleur du Temps, 261.

Couleur du Temps was a volume of short stories published by Mercure de France in Paris in 1909.

(I have not been able to find a digitised version of Régnier's Couleur du temps. Apparently, several authors published works with the same title: there is a play by Guillaume Apollinaire published in 1918, a collection of short stories by Michelle LeNormand published in 1919, a book by Jacques Bainville published in 1928 and a book by Françoise Chandernagor published in 2004.)

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Additions by MaudPieTheRocktorate:

After some searching, I found a lot more info:

From HENRI DE RÉGNIER『Couleur du Temps』（アンリ・ド・レニエ『時の色』） - 古本ときどき音楽

L’INEXPLICABLE（説明しがたいこと）

　自殺をする前に友に宛てた手紙。何も辛いこともなく、悲しくもなく、虚栄心からでもなく、酔っぱらってもいないが、これから自殺する。理由は説明し難い。ただある女性とのふとしたことがきっかけなだけだ。と旅先の中東で出会ったある女性とのほのかな恋の一瞬を物語る。

Google Translation, edited:

L’INEXPLICABLE（The inexplicable thing）

A letter to a friend before committing suicide. No pain, no sadness, no vanity, no drunkenness, and still he commits suicide. The reason is hard to explain. It just happened to be triggered unexpectedly by an issue with a woman. And it tells a moment of faint love with a woman he met in the Middle East while traveling.

I also found this strange footnote from a Chinese translation of the suicide note

法國後期象徵主義詩人。此處所指短篇小說應為〈復讐〉（當時由森林太郎，即森鷗外翻譯），多數評者指出芥川作品〈竹藪中〉（藪の中）明顯受〈復讐〉影響。

Translation

French late symbolist poet. The short story referred to here should be "Revenge" (translated by Mori Rintarō at the time, that is, Mori Ōgai), and most commentators pointed out that Akitagawa's work "In a Grove" (薮の中) is obviously affected by "Revenge".

I cannot find any mention of "Revenge" online, and just from the title, it does not look like it is a story about an inexplicable suicide.