How does power cause the tragic outcome of Macbeth?
''Macbeth'' has been called a 'tragedy of ambition'' by some literary critics, and, indeed, ambition, the quest and desire for power lies at the very heart of this play. The basic ambition is Macbeth's (his 'tragic flaw' as it were) and he wants to replace Duncan as King; and his wife Lady Macbeth is willing to help him achieve his ends ruthlessly.
As a result, Duncan is murdered under Macbeth's roof, while staying as his guest--thus violating a number of 'rules' that people believed in in those days : for example, Regicide (a king's murder) was considered a major upheaval in nature, something 'un natural' and evil; and again, the murder of a guest was a sinful thing etc.
These actions, in turn, lead to Macbeth's own tragedy, as he has violated the rules and must pay for his sins and suffer damnation--as must his wife. In literary terms, 'poetic justice' must ensue and does ensue, and in the end we have Macbeth's utter defeat and ruin.