I never liked Mortimer. Something about him just screams sleaze to me. Perhaps it's his way of declaring that he'll never accept a baseborn peasant like Galveston lording it over him. Perhaps it's just his obvious advances towards the Queen. Or perhaps it's just because, deep down, he's a total douc^h^h^h^irtbag who isn't above using a man's son to get at him. It's all too clear that he's a filthy character, even if you aren't aware of the ending of the play. Dialogs like the following are simply too incriminating to ignore:

Queen Lord Edmund, and Lord Mortimer alive!
Welcome to France.
The news washere, my lord, that you were dead, or very near your death.

Mortimer Jr. Lady, the last was truest of the twain,
But Mortimer, reserved for better hap,
Hath shaken off the thraldom of the Tower,
And lives t'advance your standard, good my lord.

Prince Edward How mean you, and the King, my father, lives?
No, my Lord Mortimer, not I, I trow.

Queen Not, son? Why not? I would it were no worse.
But, gentle lords, friendless we are in France.

-Edward II, 4.2 40-46

At other points in the play, the young Prince stoutly defends his father against both his mother and Mortimer. To put it plainly, they need him -- otherwise, they're just a French strumpet and a rebellious nobleman. But with the Prince by their side, they've got a legitimate claim on the throne that they can excercise, as the Queen makes clear :

Queen ... Ere farther we proceed, my noble lords,
We here create our well-beloved son,
Of love and care unto his royal person,
Lord Warden of the realm; and sith the Fates
Have made his father so infortunate,
Dealy you, my lords, in this, my loving lords,
As to your wisdoms fittest seems in all.

-Edward II 4.6 24-29

Disregarding the dubious legality of her declaration, it's clear that she fears what Edward would do to them all were he to retain the crown. So she uses her son as a catspaw -- since he is too young to exercise his powers, she and Mortimer, as Regents, can dispose of the realm to their liking without fear. And, as Edmund puts it:

Kent ... Edward, this Mortimer aims at thy life;
Oh, fly him then! -- But Edmund, calm this rage.
Dissemble or thou diest, for Mortimer
And Isabel do kiss while they conspire

-Edward II 4.6 10-14

Adultry, rebellion, callous disregard for the ties of family, and murder. Is it any wonder that I dislike Mortimer?