Por que esse personagem não morreu mesmo que atirou nessa pessoa em Vingadores: Ultimato?

A "resposta" (ou pelo menos a teoria sobre a qual a resposta à sua consulta se baseia) está na cena que você citou pela primeira vez. Baseado na minha única visualização do filme (spoilers, é claro):

After Stark posits this theory that the Avengers' actions in the past would not change their collective present, the other characters in the room run through a plethora of movies that instead relied on your "golden rule." In those movies, the characters' actions in the past altered their present. At the end of this rapid-fire listing of movies, Ant Man says something like, "so Back to the Future was all bullshit?" The movie then moves forward without further analysis, having laid the groundwork exposition that the past cannot be changed. The implication of all this is that, even though present-Nebula shot past-Nebula, that act did not change present-Nebula's present condition. (For what it's worth, all but one of the other events in the movie appear to conform to the theory. The one that I think may not follow the rule (SERIOUSLY, this is a SPOILER from the end of the movie, so STOP READING NOW if you don't want to spoil it!): Captain America appears at the end as an old man, having gone back in time and apparently spending a lifetime with Peggy Carter. This could have altered her timeline, unless you accept that she was actually married to Cap in the original timeline. This is a possibility as past movies did not reveal the identity of her husband. If so, I would concede this follows the in-movie rule; otherwise, it would be (TV Trope warning) New Rules as the Plot Demands.)

A teoria e o resultado do evento em seu segundo spoiler parecem desafiar a lógica. No entanto, é baseado em uma teoria amplamente aceita sobre o impacto potencial do plano que eles estavam discutindo no filme. Veja, por exemplo (aviso: passar o mouse sobre o seguinte link revelará um spoiler), Artigo do Prof. Miller na The Philosophers 'Magazine, junho 25, 2017

Parece haver duas partes nisso.

Em primeiro lugar, a citação que você mencionou, o que aconteceu no passado sempre acontecerá porque eles estão mudando o futuro e não o passado. Eles não podem mudar o passado por si mesmos, porque estão no futuro por si mesmos.

Em segundo lugar, a única maneira de mudar a linha do tempo e gerar novas, como indica o Ancião, é remover uma Pedra Infinita da sua linha do tempo. Como isso não aconteceu com o caso de Nebula, isso não a afeta.

A nebulosa principal matou uma nebulosa diferente que era de linha do tempo diferente. Por isso ela não morreu.

O filme deixou claro que Volta para o FuturoAs regras de viagem no tempo estão erradas. O significado de "o que quer que aconteça no passado, o futuro permanece o mesmo" é que o que quer que você mude no passado será levado a uma linha do tempo diferente.

Lembre-se, o passado de Prime Nebula nunca foi morto porque ela não saltou para o futuro em 2014. Caso contrário, Thanos nunca teria capturado metade da população do universo, levando os Vingadores a não inventarem a máquina do tempo, levando a não trazer de volta a Nebulosa. Isso é um paradoxo. Novos conceitos de linha do tempo são usados ​​apenas para evitar esse tipo de paradoxo.

Surprisingly this is actually how time travel is hypothesized to function. The many worlds interpretation of time travel is fairly simple. I don't have movie quotes as it's been out for only a week but pretend that what I put below is the sorceress supreme talking to Banner about the timeline:

"If you take the time stone and don't return it, the result will be that our reality will not have the time stone and therefore calamity will follow." - sorceress supreme

This is what keyed me in, especially given the explanation for magic given by the sorceress to the idea that the time travel is not time travel at all. Not really. It's actually travel to parallel universes completely indistinguishable other than that they are offset by a certain amount of time - this eliminates all paradoxes and would make Back To the Future complete horse dung as Ant Man put it. Whether this is a finite collection over some range of time or an infinite continuum is completely unknowable.

"We harness energy... drawn from other dimensions of the Multiverse... to cast spells... to conjure shields... and weapons... to make magic." - Sorceress Supreme

There are holes here of course. The existence of Captain America means that he is a Captain America from another universe where the snap occurred. He likely stayed behind when traveling to put the tesseract back. Now this means that if the year was say.... 1964 (the year before Robert Downey Jr. was born who plays Iron Man) then if the Avengers were to travel 46 years into the future they will hit the lucky jackpot of finding one universe that is not identical - the one where Loki stole the tesseract after the Hulk punched a door and that door punched Iron Man. However, this is not that same timeline or universe. It's a different universe where 55 years in the past a different universe had a snap occur and went world-hopping to defeat their Thanos and the universe the MCU exists in happens to be the universe that a Captain America and an Iron Man borrowed a tesseract from - note that this being the case proves that their victory against Thanos was inevitable so long as the other Captain America stays out of the way. I suspect that the collection of universes is incredibly muddied now that they did that. As the sorceress supreme pointed out - not putting the stones back risks the creation of parallel timelines that branch off. In other words the more they do that the more time travel actually resembles travel to parallel worlds and becomes useless.

This also explains Nebula. Nebula from the past is just Nebula from a parallel universe. It's phrased as time travel, but it really isn't and never will be in the pop culture sense.

A próxima coisa precisa de um spoiler?

TL;DR For once movies do something accurate to science.

Uma nota extra:

Once the movie comes out on dvd and I can get exact dates I might get some help somewhere and build a universe map according to this theory showing whether this continuum can actually make sense or not. The problem is that supposedly Thanos snap is an unaltered timeline and so a continuous range of universi should then go back and spawn a continuous range of Loki stealing the tesseract universes - assuming no time travel occurs this will for sure give a definitive periodic wave-like appearance to the set of timelines. I'm curious where this structure in any way prevents or contradicts its own rules.