Which attribute or quality should Hamlet have focused on to avert his tragic downfall?

Hamlet's foil was always young Fortinbras. Hamlet wanted to be him; Fortinbras is assertive, in control and "manly"! Many a time Hamlet chastises himself for thinking too much and being cowardly. Hamlet's elaborate plans to expose Claudius set of a chain reaction that becomes the tragedy that we know. Should Hamlet have pulled a Fortinbras and just killed Claudius (or at least accused him) I imagine the carnage would have been much less. It might have been messy but at least eveybody wouldn't be lying dead in the fencing hall.

Hamlet's downfall could have been avoided if he were less impulsive in killing Polonius, whom he thought was king Claudius. The murder led Claudius to believe (rightfully so) that Hamlet wanted to kill him, leading to his being sent to England, etc.