was watching this video breaking down vol. 22 of the PH manga and got stuck on this very interesting question he poses (~30 mins in):
why didn't oswald try to save lacie's life?
or, more specifically: why did oswald decide that in order to Fix Everything, he needed to kill lacie earlier rather than stop himself from killing her at all, particularly after he learned the true reason behind why children of ill omen were sacrificed in the first place (essentially to appease the jurors, who couldn't see/control them)?
to answer this, i think we have to get somewhat in-depth into oswald's motivations & relationships & POV. here's that breakdown.
first things first: oswald is a weird character. out of all of the major characters in PH's cast, and particularly those from the Past/original setting of 100 years ago, he is easily the most difficult to place or understand in terms of personality, views, etc. this is for two reasons:
jack's 100 years of intense censorship, which paint oswald as the Evil Glen Baskerville that ordered the massacre of tons of people during the tragedy of sablier.
oswald's own natural shyness and general aversion to people, which cause him to really only be close to lacie and jack (the first of whom is, of course, dead, and the second of whom does all the censorship noted in point 1).
for the entire first half of the series, we are presented an image of oswald that is a flatly evil villain, someone shrouded in darkness and mysterious red robes, in contrast w/ the image of jack as this hero of light and sparkling water and sunlight breaking through the trees. even when jack reveals that they had been friends oswald is still relegated to a position hidden in jack's shadow, true face lost to time.
our understanding of oswald therefore must almost always be taken with a grain of salt, since he is so often heavily filtered through the faulty lens jack casts on him. even so, i think we can come up with a few possible answers to respond to the Lacie Question posed above:
oswald is Panicked and Desperate to try and solve the whole "world getting chucked into the abyss problem," a SECOND time; note that he literally died in the middle of this project the first time.
in order to come up w/ the idea to save lacie, oswald would have to believe that it is possible in the first place, that there is still some Essence of Lacie that could live on. jack might believe this, but oswald definitely does not.
remember that for all the time that jack spends losing his mind coming up w/ schemes behind oswald's back to break the world's chains, oswald is mourning. frankly, he's forced to grieve lacie long before he even actually kills her. he knows, better than anyone, that she's truly dead and gone. compare these two reactions, first from jack, then from oswald:
oswald doesn't just think it's impossible for lacie to come back-- he views it as an insult, as something incredibly dangerous, to even consider the possibility of trying at all.
what i think is notable about this is that there is also yet another question hidden within the one about Lacie, which instead relates to oswald's views on his other beloved friend. specifically:
why is oswald so incredibly hurt by jack's betrayal?
again, i think we can pinpoint a few reasons behind this:
he worked behind his back for Months to come up w/ this incredibly suicidal and violent plan, even manipulating oswald's preteen (? age uncertain) niece to go along w/ his plans.
he loved jack. lacie and jack were the only two people oswald was genuinely close to. he cared about him, was invested in or even obsessed with him, and needed his support. after lacie died, he literally had nobody else-- even levi dies shortly after oswald ascends to power. jack was the only person who could possibly understand just how significant it was to lose lacie, who could relate to his grief and mourn with him-- but he went behind his back instead, spending 100 years painting him as a murderer, a villain, pinning his own crimes onto oswald's back and perverting the natural order of the world, causing his remaining family left in the baskervilles to be cast out as evil, turning chains into weapons, and generally being the biggest menace in the entire fucking series. he betrayed him in quite possibly every avenue he had the ability to. it's frankly impressive just how much he managed to stab him in the back, literally and figuratively.
HE LITERALLY FUCKING KILLED HIM
alternatively: he's not. or rather-- oswald isn't just motivated by anger or a desire for revenge against jack when he tries to go back in time and kill an even younger lacie. he is also motivated by his own guilt.
to expand a bit more on that last point, since this shit is juicy:
oswald and lacie's relationship is slightly different than gilbert and vincent's for a number of reasons, but one of the more notable ones is that oswald & lacie seem to know a helluva lot more way earlier on than gil & vince do. levi does Not fuck around when it comes to telling lacie straight up that she is going to be killed for being a child of ill omen. both oswald and lacie are thus well aware of what lacie's fate will be long before it happens, giving them plenty of time to grieve and angst over it long before oswald actually comes to power as the next glen.
due to the manipulations/information provided by jury, oswald and lacie are also taught the typical story about the origin of children of ill omen: specifically, that they are deviations that are created due to the birth of a new glen.
together, these two points lead oswald towards one bitter conclusion: that He is the reason behind lacie's existence, and therefore it is his responsibility to deal w/ her properly.
the fact that oswald & lacie are Brother & Sister and that whole gendered dynamic really adds to this point, honestly. lacie is treated like a problem child, a capricious, fickle, devil-may-care little girl that needs to be locked away in a tower for safety but breaks out to play around outside anyways. she is objectified and infantilized by most if not all of the people closest to her, treated like something dirty and evil by jury, like a literal experiment by levi, and like a inhuman goddess and representation of salvation and perfection by jack.
oswald is not freed from this-- for all that he does genuinely love his sister, he also views lacie like a Problem that He needs to fix, and takes responsibility for her actions as a reflection of his own failures. when lacie acts out, oswald internalizes that as his fault for not controlling her better. he is quite literally groomed into believing this by jury, to the point that he goes through with the sacrifice and kills her, even when he expresses clear regret and grief over it before, after, and during the ritual itself.
and on top of that, as a part of this ritual, oswald is also crowned king of the baskervilles, given an incredibly important position within the family and role in taking care of the abyss. he is more responsibility than man by the time he becomes glen, having to take care of Everyone and Everything mostly on his own, his mentor and dearest sister dying all so that he can take the fucking job.
thus: when jack starts acting out, when he loses his mind over the loss of lacie and tries to throw the world away to compensate for it, oswald interprets this as A Lacie Problem and therefore his job to fix. as incredibly unintentionally cruel as it may be, i think oswald genuinely views jack's betrayal as Lacie's Fault, and therefore his own responsibility since he is the one who supposedly Created lacie in the first place. Your Sin Is Your Very Existence-- lacie should not have existed as a child of ill omen, it was a problem that she was alive, and it's all oswald's fault for creating her so it's his responsibility to kill her properly. so he has no other choice but to forcibly take over leo's body, go back in time, and Save Everything. lacie cannot be saved because her existence itself is the heart of the issue. without lacie, jack would not be saved. without lacie, oswald would never have to meet him, or love him, or be betrayed by him. without lacie, oswald could grow up and never have to kill his beloved sister. the world's chains would not be broken, the natural order would not be tainted by a desperate man's suicidal insanity, if only the problem were not created in the first place.
and how could he possibly go through with this a second time?