To suddenly make it a rescue mission would shift the tone to a more hopeful one and I think this sudden shift would damage the movie
Isn't that what we get in the film anyways? We have a tragic, quiet death of Kirk followed immediately by another action set piece with Spock trying to kill Khan out of vengeance which than turns into a rescue mission to save Kirk. If you think having two shifts in tone is too much, I'll understand that better. But don't say that shifting tone on it's own is a bad thing, because that's exactly what the film did.
When I made the point about the tone, I made sure to add some qualifiers. I never said tone shifting was a bad thing. From my previous post, I said it's tone mismatch that would cause a problem:
Like BillJ said, it adds nothing and in my opinion would have caused some sort of tone mismatch. It had been tragic up until now and slowly descending into wrath on part of Spock. To suddenly make it a rescue mission would shift the tone to a more hopeful one and I think this sudden shift would damage the movie.
Yes there was a tone of hope in the climax but that came after
the whole issue of Spock going nuts was played out. In my opinion, what you are suggesting is this:
Tragedy (Kirk's death) ---> Hope (Rescue mission) ---> Wrath (Spock's revenge) ---> Hope (Khan's blood)
I think the hopeful tone in the middle with the rescue mission distorts the progression entirely, the audience is yanked from sorrow to hope and suddenly back to anger. Like negative, positive, negative, positive. It's almost bipolar. It will make the rescue mission feel horribly out of place.
What the film ended up doing was:
Tragedy (Kirk's death) ---> Wrath (Spock's revenge) ---> Hope (Khan's blood)
That's far more natural as it descends slowly into darkness
before it rises up into a hopeful atmosphere again.