'There would have been no difficulty if Satan were simply an Iago; the difficulty arises because he is a Macbeth too.' Discuss with relation to Paradise Lost.
One of the biggest problems or challenges in studying this poem is based around the characterisation of Satan. The fact is, as various critics have agreed down through the years, Satan is just more of an interesting character than God, and, as evil as Satan is, there is something charismatic about him that makes us enable to simply describe him as a villain that is evil to the core. The quote in your question seems to identify a parallel between Satan and Macbeth, and I would argue that this parallel consists in the way in which both characters, even though they are evil, are immensely attractive to the audience in the way that they express universal truths that are relevant to all of us.
Not only does Macbeth make some incredibly philosophical speeches in Act V, especially after his wife's death, but he also stands as an example of what happens to us if we court ambition and nothing else. In the same way, it is Satan's ambition that causes him to constantly defy God and to try and move against him. Perhaps what makes him such an attractive character is the way in which we identify so much with his character and some of the things he says. Note this famous quote from Book I:
Here at least
we shall be free; the Almighty hath not built
Here for his envy, will not drive us hence:
Here we may reign secure, and in my choice
to reign is worth ambition though in Hell:
Better to reign in Hell, than serve in Heaven.
The famous last line develops the character of Satan as an individual who will rather face damnation and perdition glady if that means he can rule his own territory rather than having to lose that power and "serve in Heaven." If we are all honest, perhaps this echoes with our own power-hungry and ambitious nature, and this in turn makes us find Satan more attractive. Satan is definitely not an Iago, who is a character that we have no connection with and feel no sympaty for. He is much more of a Macbeth, in that he sharply divides our feelings and sympathies for him.