Why didn't Lady Macbeth kill Duncan herself

Lady Macbeth says she didn't killed Duncan because he looked too much like her father as he slept. However, it's possible that this might have been an excuse to cover up her cowardice or some deeper emotional issues. Perhaps, she feels deep down that as a women she is not capable to commit a murder or perhaps Shakespheare just gave her this weak excuse to keep the focus of the plot on Macbeth and his deeds

Explanation:Lady Macbeth says that she didn't kill Duncan because he looked too much like her father as he slept. However, it's possible that this might have been an excuse to cover up her cowardice or some deeper emotional issue. Perhaps she feels, deep down, that as a woman she is not capable of committing murder; or perhaps Shakespeare just gave her this weak excuse to keep the focus of the plot on Macbeth and his deeds.

some sort of sound device i believe.