Macbeth already knew that the queen was not doing well. In act 5, scene 3, the Doctor tells him that Lady Macbeth is "troubled with thick-coming fancies / That keep her from her rest" (5.3.47-48). Macbeth, in reply, does not really display anything that looks like compassion. He essentially tells the doctor to fix it. He asks,
Canst thou not minister to a mind diseased, . . .
And with some sweet oblivious antidote
Cleanse the stuffed bosom of that perilous stuff
Which weighs upon the heart? (5.3.50-53)
In other words, he just wants the doctor to give Lady Macbeth something to make her feel better, less guilty, to make her "oblivious"—so, perhaps, to make her feel nothing at all; either way, his reply seems to me to lack any real concern for the state of his wife's health. Therefore, when he learns that she is dead, I interpret his words to be the verbal equivalent of a shoulder shrug. He says,
She should have died hereafter.
There would have been a time for such a word. (5.5.20-21)
"Should" seems to me to mean "would" here, and he means that she would have died sometime later regardless, and so the news—or "word"—of her death would have come at that time. However, it seems like all the same to him: now or later, she's dead, and the news is likely not unexpected given that Macbeth was aware of the Doctor's inability to help her.