What does Macbeth mean when he says "she should have died hereafter"?

Macbeth does not mean that he would have preferred to have his wife die later, that she "ought to" have died hereafter, or that it would have been more convenient if she had died later. He is using the subjunctive mood, which is gradually disappearing from the English language.
Shakespeare liked the subjunctive mood because it gave him poetic license. Here is an example of how far Shakespeare could go with the subjunctive mood from Mark Antony's famous funeral address in Julius Caesar.
But were I Brutus,
And Brutus Antony, there were an Antony
Would ruffle up your spirits and put a tongue
In every wound of Caesar that should move
The stones of Rome to rise and mutiny.
The subjunctive tense is used in "But were I Brutus," "there were an Antony," and "should move."