Most critics who regard Milton's Satan in a positive light, mainly the Romantics (e.g. William Blake, Percy B. Shelley), base their judgment on the glorification of Milton's Satan as hero (=the minority) in the first two books.
As Satan is preparing his first speech to the other fallen angels, the epic voice is paying respect to the enemy's more noble qualities:
84-124: "courage never tio submit or yield" (108) marks him as a true leader,
242-270: "The mind is its own place and in itself/ Can make a Heaven of Hell, a Hell of Heaven" (254-5) shows his bravery in adverse circumstances. His speech culminates in his statement that it is "Better to reign in Hell than serve in Heaven" (263). Satan is depicted as a rebel against the "tyranny of heaven", his speeches stress courage, loyalty as leader, desire to know – his "Promethean" qualities.
Satan is also full of melancholy ("tears such as angels weep burst forth") because he realizes that heaven is lost to him. One could almost develop something like "sympathy for the devil" - almost, because Satan is still full of pride and "steadfast hate", his eye is "cruel". All along, there is a level of commentary by the epic voice as a constant reminder that things are never quite the way they seem. As Milton's readers know very well, Satan is the 'Father of Lies', so everything he presents in brilliant rhetorics must be treated with utmost caution.