Why is Macbeth unlucky?
It could be argued that Macbeth makes poor decisions more than that he is unlucky, but he encounters an element of poor luck at two crucial moments early in the play. First, it is a stroke of bad luck that he discovers he has become Thane of Cawdor right after his encounter with the witches, who prophecy he will be Thane of Cawdor and greet him by that name. This leads Macbeth to put too much weight on the witches' seemingly wild prophecy that Macbeth will become King of Scotland. He starts to believe it could be true.
His second moment of ill luck occurs when, just as he has decided it would be a bad idea to murder Duncan, who is a good king and a man to whom he is offering hospitality, his wife intervenes. Lady Macbeth speaks so forcefully about her own desire to "unsex" herself and even says she would go so far as to dash her baby's brains out if she had promised to do so, that Macbeth feels compelled to go through with the murder, even though he knows it is only going to lead to more and more bloodshed. At this point, there is no going back, and it is all downhill from then on.
So, if Macbeth had not had the bad luck to run into the witches at just the wrong time, and if he had been able to evade his wife after he had decided not to murder Duncan, he might have had a much better fate.