Of course, as Gillan points out in her comments to Digital Spy, simply surviving the first film didn’t mean that she was guaranteed to appear in the next one.
Karen Gillan doesn’t go into detail about exactly what happened to Nebula in the original script.
In the final version, Nebula lost a hand in order to escape.
However, we begin to get an idea why Nebula was allowed to live, as we know that the relationship between the two sisters will be a significant part of the story of Guardians pf the Galaxy Vol. 2.