One critic said, "Macbeth’s downfall is complimented by his changing views on masculinity and how it is presented through ambition, morals, patriotism, gender, and ultimately violence."
If this is the case, then I think Lady Macbeth is certainly a trigger for Macbeth's masculinity issues and, to a lesser extent, his ambition, violence, and immorality. In her famous soliloquy, we hear of Macbeth before her influence: she says he is "too kind" when it comes to his friends, kinsmen, King, Scotland. She knows he is certainly not kind to his enemies, as we see in the Bleeding Captain's account. So, she plans to work on convincing Macbeth that his kinsmen are his enemies. She plans to trigger his cruelty by appealing primarily to his masculinity:
I fear thy nature;
It is too full o' the 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: thou'ldst have, great Glamis,
That which cries 'Thus thou must do, if thou have it;
And that which rather thou dost fear to do
Than wishest should be undone.' Hie thee hither,
That I may pour my spirits in thine ear;
And chastise with the valour of my tongue
All that impedes thee from the golden round,
Which fate and metaphysical aid doth seem
To have thee crown'd withal.
Macbeth is cruel throughout the play, but Lady Macbeth convinces him to turn his cruelty toward Scotland, and she appeals to his fear of being too soft and feminine to do so. As soon as he entertains her murderous plan, he starts to suffer from madness: hallucinations, hypersensitivity to sound and time, guilt, megalomania, delusions, and boundless cruelty. Later, she will try to use the same appeal to his masculinity to try to save him from madness. During the banquet scene, she says,"Are you a man?" By then, it is too late. Lady Macbeth should have been more careful in what she asked for because she indeed helped create a monster.
No, Lady Macbeth is not the trigger that shoots off Macbeth's madness. We first see his madness in the manner in which he butchers MacDonwald in the beginning of the play. And then we see more of his madness when after receiving the gracious reward of Thane of Cawdor from King Duncan, he has thoughts of assassinating Duncan. These are not actions and thoughts of a sane man, especially the latter, for he has just won the war for Duncan and decapitated the head of the traitor. He has proven himself a loyal servant of the king, and now he is entertaining the notion of murdering him.
Macbeth suffers from schizophrenia; this is revealed when he sees the floating dagger that leads him to Duncan's chamber. And later in the play Macbeth admits that he is insane when he says "O full of scorpions is my mind."
If anything, Macbeth uses Lady Macbeth to spur his intentions.
The text suggests that Macbeth is predisposed to seeing visions and illusions. This is an aspect of mental illness. Since the audience doesn't get to know Macbeth much before he kills Duncan, one can argue that the act of murdering Duncan either triggers or exasperates the traits of madness or mental illness that Macbeth already possesses. While Lady Macbeth may have encouraged Macbeth to kill Duncan through intimidation and coercion, she is not the direct trigger.