Eustace is ironically termed "sly" and "cunning" the very moment he shows he is not. His stock exchange: "I, the influential brother of Godfrey, will help you, Richard, become the new chief of the mercenary troops, and you will let me follow Armida in this enterprise" is ridiculous, and worse than that, for a number of reasons:
1. Richard, who is not a fool, will immediately understand his clumsy try.
2. Eustace, like many others 'blinded by love,' can't see how dangerous Armida is; her request is a trap, she is not only a witch (like in Gerusalemme Liberata), she is not even human (a hybrid between a man and a mermaid).
3. Eustace doesn't know, nor do we at this point yet, that Richard (unlike Rinaldo in Gerusalemme Liberata) is mainly gay, so he is not specifically interested in escorting Armida.
4. And nonetheless the handsome and valiant Richard, not Eustace, will have a bed story with her.
5. Richard accepts Eustace's pact and will get that role, but precisely this will cause a lot of troubles, so much so that the success of the whole Crusade will be jeopardized.
All this clearly shows, incidentally, that the great epic poems were built so as to be fully appreciated only on second, or third, etc., reading, though very enjoyable from the start.
Interesting is also the process of psychological operations as described here with reference to Eustace: (a) a deep, irrational feeling arises [jealousy in this case], which (b) inspires (c) the heart, conceived as the center of decisions, and just then (d) thoughts proper are formed.