Hamlet's inability to act is indeed his tragic flaw, as this is the quality that causes him pain throughout the play. Hamlet knows that he has to avenge his father, and he desires to do so, however, he is incapable of acting. This gives Claudius the chance to plan the demise of Hamlet. Moreover, Hamlet is indecisive about Ophelia. While sometimes loving her and something ignoring her, he cannot make up his mind about her, which leads to her ruin and eventual death. Hamlet's inaction contrasts with the decisiveness of Laertes and Young Fortinbras, who both effectively take action to avenge their respective fathers (although their ends are significantly different).
