Macbeth’s main concern is gaining and keeping the throne of Scotland. He desires power, and when he has it, he wants to make sure no one else gets it.
That one overarching concern fuels several other worries. In the beginning of the play, he worries about killing Duncan, but his ambition gets him past his fear. He frames the guards and Duncan’s sons, so Macbeth does not have to worry about being blamed for the execution.
But once he is king, he worries about maintaining his power:
“To be thus is nothing;
But to be safely thus.”
He also focuses on the fact that the witches said Banquo’s descendants would be future kings of Scotland, not Macbeth’s. He begins to wonder why he bothered to kill Duncan if he would not have his own line succeed him on the throne. So he plans the deaths of Banquo and Fleance.
Still he is not at ease. Macbeth consults the evil witches once more, who use more misleading predictions to cleverly convince him that he is invincible, including the fact that no one ‘born of woman’ could harm Macbeth, and he couldn’t be vanquished until Birnham Wood marched to Dunsinane hill. Macbeth feels this is confirmation that he will retain his great power, since isn’t everyone born of woman? And trees can’t walk. His desire for power has ensnared him even more deeply with evil.
When Macduff, the Thane of Fife, doesn’t come to the banquet at the castle, Macbeth suspects Macduff is disloyal. When it is confirmed that Macduff has gone to England to raise an army against Macbeth, Macbeth orders the murder of Macduff’s wife and their small children:
“The castle of Macduff I will surprise;
Seize upon Fife; give to the edge o' the sword
His wife, his babes, and all unfortunate souls
That trace him in his line.”
Macbeth murdered to gain power and kept killing to keep it. His ambition turned him into a complete tyrant.