In nu trek everything is so simple and convenient, it's scary.
Like why was it so important that Spock didn't kill Kahn?
First the blood takes some time to die after someone has been killed by several blows to the head. Long enough to beam the dead body to sickbay and extract whatever blood is necessary to revive Kirk.
Second, if not then they had all the other popsicles, didn't they?
So no, Spock could have killed Kahn if he wanted too, but it was just convenient that he did not.
...if we go by your description (which I don't. You're ignoring other facts to make the argument work), then keeping Khan alive was the very opposite of 'convenient.'