“As I have already had occasion to relate, he was angry at finding himself reduced to chopping logic about this young lady; he was vexed at his want of instinctive certitude as to how far her eccentricities were generic, national, and how far they were personal. From either view of them he had somehow missed her, and now it was too late. She was ‘carried away’ by Mr. Giovanelli.”
If the use of ‘either’ refers to any of the three explanations above (generic, national, personal) (OED I.4) then the narrator offers three explanations for Daisy’s behavior: that it is so generalized as to be unrelated to her intent or motivation, that it is a national characteristic of Americans, or that it is a personal trait which she exhibits to a specific and unusual degree. If ‘either’ groups ‘generic’ and ‘national’ into one set, and ‘personal’ as the second option (as is perhaps suggested but not convincingly proven by the division of “how far…” across the list) then national traits become generic—but only, as the text proves by Daisy’s ostracization, explicitly noted earlier in this passage—within the physical space of that nation (it cannot be among all members of that nation or Americans like Winterbourne and Mrs. Walker would not be so scandalized (yes, you could argue that “we have been here too long” negates the suggestion of them possessing American points of view… but then why does Mrs. Walker continue to study Europeans “like textbooks” at her parties? If her point of view has been shifted to be entirely Eurocentric and European-esque, then continued study would be unnecessary. One could more convincingly argue that Winterbourne and Mrs. Walker represent, to different degrees, an entirely different point of view—that of the transatlantic multinational. But that’s a different blog post). The final way to read the “either view of them” is to see ‘either’ as referring to one or the other of Daisy and Winterbourne. I find this third reading the most compelling—the most common use of ‘either’ is in distinguishing between two things (or people) (OED I.1) and the rest of the sentence supports the ‘them’ allusion to the two characters: “…he had somehow missed her, and now it was too late.” ‘He’ and ‘her’ would therefore make up the ‘them’ earlier in the sentence.
If we move forward with this third reading then the contrast provided is between two “view[s] of them [Daisy/Winterbourne]” and this means that the narrator acknowledges that, depending on which “view of them” one takes, there are different positions for the reader to position themself within. It would also mean that regardless of which of these two views the reader experienced the text from (the Winterbourne-centered view or the Daisy-centered view) in both of them, the narrator acknowledges that “he had missed her”. This is the first time the passage entertains a “he…her” construction—earlier in the passage the structure is either reflexive (Winterbourne saying to himself, asking himself, angry at finding himself, etc.) or set up between the narrator and Winterbourne (“it must be admitted that holding oneself” “came to seem to Winterbourne”). The only direct action between Winterbourne and Daisy—that of him missing her—lays at Winterbournes feet as his own failure: whether the Winterbourne or Daisy-centric view of them has been adopted by the reader, the action is the same, and who (he) missed who (her) is the same; “and now it was too late.”
Winterbourne is the one who dropped the ball, the one who “missed” Daisy, regardless of whether you read the novel considering Winterbourne or Daisy to be at its center. So what? So ultimately the story is of “missed” opportunity and “missed” understanding on the part of Winterbourne, and it resulted in death (the next line explains that Mr. Giovanelli has now carried Daisy away). We were searching for a moral within this novel: if you squint, and splash around in the OED a bit, you can find one in the final sentence of this passage.