Absolute Anime

Ok if Trunks went back to the future and killed Cell before he got into the time machine and while in the past he killed the other Cell in the lab would that not have changed the history more shouldn't Goku be alive

This is where it gets complicated. I read anout a theory about multiple universes created by time travel in a Terminator FAQ. Imagin going back in time and stoping your parents from meeting. If it is only one universe with one timeline you would cease to be. However the theory goes you were concieved in a reality were your parrents did meet. You cannot alter the history of that reality. You merely create a new one. (This means you would still exsist even if you stopped your parents from meeting as you were born in one reality while creating another.)

If the above makes any sence then Future Trunks and Cell are from different realities created by variants in the timeline. When Trunks mentions his future would not be altered it is because that reality is already set.

I hope this helps.

i thuoght so but if true happened would what was the perpose of coming but in time shouldent he have realized that after killing the cell in the past

the purpose was to has a new saga

Also, I the future, doesn't everyone else die, or am I confused?

the future trunk's mother told him that she wanted to create a better future so after trunks goes back and alter time a new parallel universe is created one better than the time trunks comes from so the future bulam' wish is fulfilled...

so does she want to make a new machine that came go through dimenions