You want to reframe the prompt so that you don't have to talk about extent. 'Otherwise this would be a recipe for a convoluted contention culminating in a disastrous essay.' to quote my teacher last year.
The example he gave was: 'Richard III is totally evil, but the other characters in Richard III are not much better.' Discuss.
This one doesn't actually use the word 'extent' but it is difficult to discuss the prompt directly without the putting the characters on a sliding scale from 'Totally Evil' to 'Not Evil'. You don't want to be talking about types of evil or suggesting that only Richard is evil, or to go off track by talking about justifications for evil, which might make it 'less evil'.
Your thesis should still be complex enough to encompass the idea that this isn't a black and white thing, but it should be framed in a manner such that you can make comparisons and juxtapositions between characters and avoid falling into the trap of talking about 'matters of degree'.
Example (not a great one but it will do I think, especially since RIII isn't on the course anymore anyway.):
"Evil permeates the minds and actions of many of the characters in Richard III. However, it is counterposed by the crises of conscience and guilt to which even Richard falls prey to, and by the presence of the female triad in the play. Although Shakespeare molds Richard in the likeness of 'the formal Vice, Iniquity' as Richard himself points out, his evil is unable to supplant his underlying humanity. Few of the characters in Richard III are arguably good, but of those that act evilly, either before the action of the play, or beforehand, all are punished, and some even repent."