I mean why not say "For what it's worth I had hoped we could avoid this day"?
Because they chose not to.
The meaning of the original and your suggestion is pretty much identical.
So too would be, "I hoped we could avoid this".
Could have does put the focus more on the meaning "something that was possible, but which did not happen".
However could while not having that meaning so strongly, implies it in this phrase so much as to be synonymous: It's possible to say something like "I could avoid this day, so I made sure I did", but here there's no ambiguity, so they're both equally valid choices.
And we have equally valid choices all the time. We have "To be or not to be" vs "Maybe I should just top myself", through to much less extreme differences.
We have our own natural styles that lead us to one decision or another. It may be that this author is more likely to favour "I had hoped we could have avoided". over "I had hoped we could avoid" as part of their natural style.
We also push our styles in different directions to use different voices in different registers and contexts. This is even more so when we are writing dialogue. It could be that the author would never say something like "I had hoped we could have avoided", but they are picturing the character as someone who does. The rest of the phrase is wordier than necessary to convey the bare meaning in other ways, so the choice here is of a part with those other stylistic choices.
It would seem to be at a point where the killer is explaining themselves, and no doubt about to attempt some violence upon another character. This is a reason for a writer to draw out speech both to show hesitation and woolgathering on the part of the character, and to keep the reader at this point of suspense a bit longer. Therefore even if the character is one who would normally just say "I hoped we could avoid this", it could be an attempt to benefit both the sense of the character's current mental state, and the pacing of the story, to favour the longer form.
And that's all assuming the best on the part of the author. Maybe they just sat down and typed!
I'm inclined to suspect it was at least semi-deliberate though, since it's a bit of a cliché that the villain be a bit wordier than usual. Now, that cliché being so common is a good reason to rephrase in itself, but it's also a likely motive on the part of the writer. (It's also more forgiveable in earlier texts, before the wordy murderer became so over-used).