third time a charm

"Edward III: Think not that I am frighted with thy words.

My father's murdered through thy treachery,

And thou shalt die, and on his mournful hearse

Thy hateful and accursed head shall lie,

To witness to the world that by thy means

His kingly body was too soon interred.

Queen: Weep not, sweet son.

Edward III: Forbid not me to weep. He was my father,

And, had you loved him half so well as I,

You could not bear his death thus patiently.

But you, I fear, conspired with Mortimer." V.vi.27-37

Act 5 presents the emergence of Edward III as a shrewd and capable leader.  History provides an account of Edward II depicting him as a foil to his father, the decisive Edward Longshanks. The crowning of Edward III restored a "kingly character" to the throne which had been absent since his grandfather's reign. And while Marlowe's play shows the arrest and execution of Mortimer Jr. taking place immediately upon he being named king at 14, Edward III in actuality was made to wait 3 years, until he was 17, before he was able to exact his revenge. 

One is left to wonder throughout the play what meaningful contribution Edward II will make to England, and the concluding scenes make evident that the answer is his son. This significantly adjusts the reader's perspective on the central character of Edward II; no longer are we left to look with pity and distaste on his fanciful sovereignty, but now may view his existence as a vehicle delivering England the rule of Edward III. Until the final events of the play, one might argue that Mortimer Jr. and the Queen--in working against and eventually depositing the irresponsible and self-absorbed King--are doing what is best for England. Act 5 firmly denounces this notion, however, assigning sympathy and indirect justification to Edward II, as well as undeniable villainy to his conspirators.