Reading this quote in context, it comes after Satan has mounted the tree and has seen with wonder and despair the form of Adam and Eve, in whom "so lively shines... divine resemblance." As he sees them he plots their downfall and declares that "Hell shall unfold / To entertain you two." However, at the end of this speech he seems to offer the "tyrant's plea" to excuse himself for his actions in planning the downfall of the pinnacle of God's creation:
And should I at your harmless innocence
Melt, as I do, yet public reason just,
Honour and empire with revenge enlarged,
By conquering this new world, compels me now
To do what else though damned I should abhor.
This, I think, is the "tyrant's plea," with Satan being the tyrant, yet feeling the need to "excuse his devilish deeds" by saying that he is compelled to pursue his plans to corrupt the innocence of man, although if he were not "damned" he would "abhor" such a plan. Satan as a character is absolutely fascinating in this entire epic classic, and the way he presents himself is worthy of discussion. Here we see his apparent need to excuse his behaviour. The reference to a "tyrant's plea" almost appears to be paradoxical, as tyrants by their very nature do not make pleas, thus adding new levels of complexity to the character of Satan.