8 "Unanswered Questions" From Avengers: Endgame, Explained
5. Why Didn't Nebula Or Tony Tell Clint And Natasha What Getting The Soul Stone Would Involve? They Knew Thanos Murdered Gamora On Vormir.
There is no explicit mention of why Thanos kills Gamora given to other characters in Infinity War. Peter Quill accuses Thanos of lying about Gamora's death, which Mantis disproves upon feeling his emotions and telling the group that he is "mourning." For all these characters know, Gamora could have attempted to get in Thanos' way at the last moment, leaving his only option to murder her to get to the stone.
Nebula does not know what Vormir looks like or what is there. No one else has ever found or come into contact with the Soul Stone in the history of the universe. All she knows is that Thanos murdered a captured Gamora there and then emerges with the stone, which is the information she accurately relays to the group when planning the time heist in Endgame. It is Natasha who eventually draws the conclusion between the two facts once she's on Vormir herself.