JRRT really was constantly ping-ponging between which of the two brothers should fall in love with Luthien, lmao
"In Synopsis II, given on p. 233 to the point equivalent to the end of Canto VII, the plot reaches almost to its development in the present Canto of the Lay; but this was achieved in stages, and the original text of the outline was so much changed and extended by later alterations that it would be extremely difficult to follow if set out as hitherto. I give it therefore in two forms. As first written it read: "Curufin and Celegorm go hunting with all their hounds. Huan the sleepless is the chief. He is proof against magic sleep or death - it is his fate to be slain only by the 'greatest wolf'. They espy Luthien who flees, but is caught by Huan whom she cannot enchant. The hound bears her to Celegorm, who learns her purpose. Hearing who she is, and falling in love with her he takes away her magic cloak, and holds her captive. At last he yields to her tears to let her free and give her back her cloak, but he will not aid her because of his oath. Nor does he desire to rescue Felagund, since he is now all-powerful in Nargothrond. She departs from Celegorm. But Huan has become devoted to her, and goes with her." At this stage, the hunting evidently had no significance in itself: it was the device by which Huan (already in Synopsis I the hound of Celegorm, and with a peculiar fate) was to be brought to accompany Luthien, an essential feature going back to the Tale of Tinuviel. There is no mention of her being hurt by Huan, as there is in Synopsis I (and so no question of 'redress'); and here Celegorm falls in love with her and therefore holds her captive. But this is only for a time; he yields to her prayer and gives her back her cloak, though because of his oath he will not aid her; and the evil motive of his desiring to let Felagund perish so that he may retain power in Nargothrond appears. Luthien leaves Celegorm; Huan goes with her, as in Synopsis I, but the motive is now explicitly the hound's love for her. After emendation the outline read as follows: "Because of the disguise of Felagund Thu is suspicious and his wolves fare far abroad. Celegorm seizes pretext for a wolfhunt. Curufin and Celegorm go wolf-hunting guilefully (really to intercept Felagund*) with all their hounds. Huan the sleepless is the chief. (Huan came with him [i.e. Celegorm] from Tavros' halls.) He is proof against magic sleep or death - it is his fate to be slain only by the 'greatest wolf'. They espy Luthien who flees, but is caught by Huan whom she cannot enchant. The hound bears her to Celegorm, who learns her purpose. Hearing who she is, and falling in love with her, Curufin takes away her magic cloak, and holds her captive. Although she tells him Melian's words and that Felagund and Beren are in Thu's power he won't attempt a rescue even of Felagund. (Marginal note: It is Curufin who put evil into Celegorm's heart.) In spite of her tears to let her free and give her back her cloak he will not aid her because of his oath and love. Nor does he desire to rescue Felagund, since he is now all-powerful in Nargothrond. But Huan has become devoted to her, and aids her to escape without her cloak.
The History of Middle-Earthย #3, "The Lays of Beleriand"
So, at first it was Celegorm, but in that version his love managed to awaken something sincere in him, and he let Luthien go, even though it wasn't a true redemption and he still wanted Finrod to die. Then it was Curufin, who was the full-blown villain from whom Huan had to protect Luthien ("Curufin thereafter never near might win to Lรบthien, nor touch that maid, but shrank from Huan's fangs afraid")
And it seems the final decision was made only much later, after The Lord of the Rings, when Tolkien decided that Curufin already had a wife.
Now, in the published Silmarillion, we seem to have a sort of combination of those two earlier versions: it's Celegorm again, but he doesn't let Luthien go, isn't moved by her situation at all, and thinks only about the power he would gain through marrying her. And when she escapes, both he and Curufin would rather try to kill her than allow her to get away.
