The fact that Macbeth conceals information from his wife can be interpreted in two basic ways. One interpretation is that he is trying to spare her feelings because he thinks she will be upset about killing their allies. This interpretation is implied because he mentions that she should remain “innocent.” A reason to doubt this, however, is that in her earlier advice to him, she had mentioned looking like an “innocent flower” to disguise that one is acting like a serpent. Therefore, he might be saying that she should pretend not to know what is going on around her, rather than his just wanting to conceal this information.
Another, different way to look at his caution is that he does not quite trust her to back his plan. Up until this point, they have operated hand in hand, but it has been Lady Macbeth goading him to action. The blood on his hands seems to be motivating Macbeth to commit further violence. He may understand the need to keep his plan secret from his wife because he knows deep down it is not a good plan, and she will try to talk him out of it. In this line of thinking, as up till this point they had succeeded because they backed up each other, this breach of confidence marks the beginning of both their downfalls.