A tragic impulse is something that leads a character to act in a destructive and self-destructive manner. In Macbeth, both Lady Macbeth and Macbeth desire power but have no way to achieve it. Each of them acts in a way that is destructive to others and him or herself.
Let's consider Macbeth. Macbeth shows plenty of ambition before his wife even comes into the picture. He is clearly thrilled that the witches prophesize him as king, because when Malcolm is named successor he is angry and vows to make himself king anyway.
The Prince of Cumberland! That is a step(55)
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires (1:4)
Macbeth does not care that he actually has no legitimate claim to the throne. The idea has been dangled in front of him, and he feels he has to take it.
Macbeth cannot become king in any other way than killing Duncan and getting Malcolm and Donlbain out of the way. Then he has to also kill Banquo and Fleance to ensure that he does not have a “fruitless crown” and there is no threat to his power.
Macbeth begins to unravel though. He suspects everyone. Since Macduff did not come to the banquet, Macbeth is concerned that he may be acting against him. He decides the only thing to do is kill Macduff.
I hear it by the way, but I will send.
There's not a one of them but in his house
I keep a servant fee'd. (3:4)
Macbeth has revealed that he has spies in Macduff’s house, and likely has spies in other nobles’ houses. He sends murderers to kill Macduff’s wife and son. He knows he is not there! It is a heartless act. This tragedy leads to tragedy for Macbeth, as his wife can no longer take the guilt and kills herself, and then Macduff kills Macbeth in battle.
The desire to see into the future and affect it proves to be Macbeth’s downfall. He tries to control too much. He uses the weird sisters to see what will happen. As a result, he thinks he is safe because the forest is not going to come to Dunsinane, and no man born of woman can kill him. He is so blinded by his need for control that he does not see the real danger.