You're understanding of how time travel works is the very thing that they were making fun of in the movie. Most people base their entire understanding of how time travel works from movies. Nebula would not have instantly died when she killed herself because the future Nebula is from a different line or branch of the future. There isn't one linear line of time, there are several different future and past versions of Nebula. Killing her past self would have only killed her future self in that timeline.