The dominant theme of Edward II is the theme of many of Marlowe's (and Shakespeare's) histories: the will to power and, ultimately, the corruption inherent in power. Edward isn't thwarted and murdered because of his affection for Gaveston. Rather, it is because in bestowing such extravagant favors on Gaveston, a commoner, he is subverting the "natural" order of his position, neglecting both his kingdom and his family. He comes to realize, too late, that his arrogance and his disrespect of himself - or, more precisely, the institution he represents: the monarchy - has cost him his love (Gaveston) and his life ("What is a king but a shadow on a summer's day?') But the theme is carried on through the machinations of Isabella and Mortimer. True, she may be seen as a "wronged wife", but her revenge in collusion with Mortimer is also a subversion of the "natural" order. She is willing to commit both murder and regicide to achieve her ends and she, too, is thwarted by her son when he assumes Edward's throne and puts her away. The tone of the play suggests that Marlowe saw the will to power - whether or not it was legitimate - as something of a vicious neverending cycle and, in this respect, his vision is much darker than Shakespeare's. You might find it useful to compare Edward II to Marlowe's later (and lesser) work, The Massacre at Paris.