It is hard to get one quote to sum up the whole play but if I was choosing one I would go for:
And oftentimes, to win us to our harm, The instruments of darkness tell us truths, Win us with honest trifles, to betray's In deepest consequence...
said by Banquo in the opening Act, scene 3.
It links to the whole idea that an honest and basically man, Macbeth, is corrupted and led down an avenue of destruction after his meeting with the witches- or wierd sisters as they are called. It fits in with the belief of James 1st of England in witches and the fact that they can influence what happens in the world for ill. Macbeth is obviously an ambitious Lord, but at the start he is loyal and rated highly by his peers. The opening acts begin to show the trail of decisions that will lead to his destruction, and that of his wife. If you read the summary in the e.notes version of the play you will get a good overview. But basically the opening three acts show Macbeth and Banquo as the heroes, then the witches appear and give the prophesies. Macbeth sends a letter to his wife who is determined he will murder King Duncan and not leave it to chance that he will become king. Killing leads to killing and Macbeth attempts to eliminate Banquo and Fleance, he succeeds to kill Banquo, but Fleance will live. Macbeth moves from being surrounded by friends and good will, to being suspicious, alone and although in power clinging to it and using that power in a very negative sense. He is haunted by bad dreams and has lost his peace of mind. This is why I feel the above quote sums up the message of the play.