What does Lady Macbeth fear will be an obstacle to the commission of the crime and what does she decide to do about it?from act 1 scene 5 of Macbeth
If we look at Act I scene 5 carefully, we can see Lady Macbeth's response to hearing about the prophecy of the witches and her concerns about what might prevent Macbeth from gaining the crown that the witches have promised him. The answer to your question can be found in the soliloquy that Lady Macbeth gives:
Yet do I fear thy nature:
It is too full o'th'milk of human kindness,
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it: what thou wouldst highly,
That wouldst thou holily; wouldst not play false,
And yet wouldst wrongly win...
Lady Macbeth then obviously sees the potential for gaining power and prestige, but she is also aware of her husbands "failing" in that he is too overcome by moral scruples to do anything untoward that might help the prophecy come to life. So it is that Lady Macbeth wants her husband to come back swiftly so that she can "pour my spirits into his ear" and chastise and encourage him to do what she feels needs to be done to help the prophecy come about.
Fundamentally, Lady Macbeth fears Macbeth's sense of honor in preventing him from killing Duncan. She discloses her desire for power and in doing so, the reader/ audience understands how Macbeth is not comfortable in doing what is being asked of him. To this end, Lady Macbeth goes to several lengths to overcoming the obstacle of his honor. The "unsex me" speech is one such demonstration of this, in that Lady Macbeth wishes to be "more of a man" than Macbeth could be. At the same time, she gives him a sort of "road map" on how to overcome his own sensibilities in his "honor." She recognizes that Macbeth lacks the vocabulary and the pattern of recognition to fully understand how to do what is being asked of him. To this end, she advises him to "beguile time" in presenting himself as the "innocent flower" while becoming the "serpent." In telling Macbeth how to act duplicitous, Lady Macbeth strives to overcome Macbeth's inherent loyalty and honor as a subject and covet what she wants and what she feels, to an extent, what he wants as well.