Satan's defiance and his desire for revenge overcome his pain. At first he seems dismayed as he addresses Beelzebub, once like him among the brightest angels and now "O how fallen!" But as soon as he speaks of God, "He with his thunder," Satan's rage overtakes his sympathy. He will not repent or change. "All is not lost" while he has his "unconquerable will / And courage never to submit or yield." He will continue the war, either by force or by guile. Because we know the story of Adam and Eve and how Satan will corrupt them, "guile" is like a wink at a knowing audience.
You may think that Beelzebub takes a more realistic view of the fallen angels' terrible situation because he thinks further rebellion is futile. He regrets what has happened. The fallen angels may feel their strength undiminished, but perhaps God has left them that strength only so that they can work as slaves in Hell and has allowed them their immortality so that they can feel acutely their eternal punishment.
To do aught good never will be our task, But ever to do ill our sole delight.
Then he draws Beelzebub's attention to the fact that God has recalled his forces and left the fallen angels to suffer in Hell. Things now seem calm enough for them to leave the lake and hold a meeting of their troops on a "dreary plain," to plot their revenge strategy.