When receiving the news about the prophecies, Lady Macbeth wasted no time developing a plan for Macbeths domination over the throne. She tried to suppress her feminine qualities in order to show no mercy. Knowing that she could not do it by herself, she manipulated Macbeth into committing her murder. She steadily lost power over him after Duncan's death but she was still the reason behind the shift in Macbeths honest and honorable mentality.
Both had to endure guilt and many other negative emotions rising from what they did. Lady Macbeth was not able to handle it and broke down completely, this was selfish because Macbeth was going through the same thing and needed her to keep him afloat. By killing herself she pulled the final straw from Macbeth sanity. From that point, his morals had collapsed.
Many factors could have been blamed for Macbeths demise, but without Lady Macbeth in the mix, King Duncan's murder and the events proceeding would have not taken place. Macbeth needed her initial push, and he only accepted it due to their marriage. The witches would have never convinced him, to him they were old hags, with no connection. Macbeth himself wouldn't have decided to attempt the murder because of his nobility. He needed a push from someone he trusted most, and Lady Macbeth was there for him. Without Lady Macbeths initial spark, Macbeths bloody path would have not lit up.