How we cite our quotes:
"so bent he seems
On desperate revenge that shall redound
Upon his own rebellious head" (3.84-6)
In Book 10, Satan's plans for revenge will "redound" upon him, just as God says it will here. Notice the use of the word "bent." Good characters – Adam, Eve, Abdiel – are often described as "upright" or "erect." Satan, in contrast, is "bent" on "revenge," a phrasing that suggests a connection between revenge and something opposed to uprightness.
"yet public reason just,
Honor and empire with revenge enlarged
By conquering this new world compels me now
To do what else, though damned, I should abhor" (4.389-92).
Satan's subservience to "public reason" – probably some sense of duty or responsibility to his legions – is what partly causes him to go through with his plans of revenge. He suggests that the only reason he's still going through with it is because he made a promise. He makes a distinction between a "public" and a more private self, crediting all his evil plans to the former and all the nicer ones (about abhorring what he's about to do, melting at the sight of Adam and Eve right before this, etc.).
"But what will not ambition and revenge
Descend to? Who aspires must down as low
As high he soared, obnoxious first or last
To basest things. Revenge at first though sweet
Bitter ere long back on it self recoils" (9.168-72).
Satan's words reveal a bitter irony; his revenge will quite literally "back on it self recoil" in the next book, when he and his companions are changed into serpents. The same is true of his remark about how one who "aspires must down as low." Satan tried to soar to the top (of God's throne) but ends up in Hell, a place at the bottom of the universe both literally and figuratively.