What about Macbeth does lady macbeth fear will prevent him from committing the murder?

Lady Macbeth is afraid that her husband is too kind and caring to kill King Duncan. She says in her soliloquy of Act I scene V-

yet do I fear thy nature;

It is too full o' the milk of human kindness

To catch the nearest way:

She says that he is certainly ambitious, but does not have enough evil in him to kill in cold blood-

thou wouldst be great;

Art not without ambition, but without

The illness should attend it: