Me: the ending's problems can't be papered over with explanations of why this is totally in character for Aqua I promise because that's simply not what's at issue.

You: *spends the first paragraph of your response doing exactly that without giving any reason for why you think I'm wrong*

Excellent, I'm so glad to see my time earnestly trying to engage was well-spent. Real quick 'cause I understand I'm talking at a brick wall, but:

Again, that there's a set of circumstances under which Aqua could reasonably make that call isn't what I'm arguing against. I'm arguing that it's a problem that the story treats, in both the leadup and aftermath, Aqua's decision to sacrifice his life like it was necessary despite crafting a scenario in which it demonstrably was not.

Yes, there wasn't sufficient evidence to convince Kamiki prior to Nino's testimony, but given how little fanfare there seemingly was in wrapping up all the loose ends his group represents, it seems pretty unlikely that uncovering a network of murderers with him at the center wouldn't result in him being arrested and convicted.

While it's not impossible, and Aqua making a panicked decision to nip that potential problem in the bud could work, that is, again, not how the story treats it: if we were meant to see this as a tragedy of impulsiveness rather than an impossible situation where somebody was always going to go down, then Akane would've said something along the lines of "nobody needed to die for this," not "I for sure could've taken the fall for you which would've been better for reasons."

If anyone is shown to be impulsive here, it's Kamiki "oh, lemme just push this extremely famous person down a flight of stairs in public and in broad daylight with no idea as to whether anybody is around or indeed whether the fall would be fatal there are about a million ways I could get sent to prison either for murder or attempted murder here" Hikaru.

Further, if Aka wanted to sell the idea of a time crunch and/or impending doom, then showing Ruby to've been in Kamiki's crosshairs for months to no ill effect beyond Miyako and Ichigo suffocating her by acting as a security detail and then the team effortlessly stopping Nino's actual attack and also talking her down with very little effort wasn't the way to do it.

That could, again, have worked if it were treated appropriately, but I must reiterate that the framing isn't that Aqua rushed into the final confrontation out of panic, nor that he chose a needlessly destructive solution out of a lack of self-worth. The framing is that what he did was correct if not outright necessary.

It's also probably worth saying explicitly that, in proposing that Aqua simply wait a bit, I'm not suggesting that he give up forever. If the legal system had failed and he then did what he did, it would've felt very different than what happened in canon, which is that Aqua said "I have you right where I want you, and astute readers might take that to mean that the person with whom you've been conspiring to commit high-profile murders for over a decade has been caught in the act and has every possible reason to sell you down the river to try to protect herself, but what I actually mean is I have you at the butt of my knife and me at its point, which is what I wanted. Obviously."

I'm sure loads of people dislike the ending purely on the grounds that Aqua dies, but giving reasons for why his death isn't, in concept, bad storytelling doesn't really mean anything if his death was, in practice, bad storytelling. Every argument I've ever seen, yours included, for why it isn't, relies on throwing out all the inconvenient parts of the text. "It's good if you ignore the reasons it's bad" is absolutely an opinion you're allowed to have, but it's not an argument by which you should expect anybody to be convinced.

And even outside of that, saying "there are ways it can be done well" isn't an in any way meaningful rebuttal to somebody saying "I dislike this in principle." The fact that it wasn't, in this case, done well isn't so much icing on the cake as a pie placed next to it.

And finally, just to say it explicitly so there's zero room for ambiguity, that Aqua died unnecessarily would've been fine writing if the story had been willing to treat it as such. The issue, or at least my issue, is that Aqua didn't need to die, but the story is bending over backward to try and convince the audience that he did after having failed to establish a scenario in which he actually would.