I think it would be naive to suggest that Pope Clement VII acted from the purest of motives. His decision to deny Henry VIII an annulment was partially based, no doubt, in his fear of the Holy Roman Emperor. There were, after all, theological doubts relating to the validity of Henry's marriage to Catherine; she had previously been married to Henry's elder brother, Arthur.
I was under the impression Henry had to seek special Papal dispensation to marry Catherine, so how he could then turn around and claim some years later his marriage was annullable on the basis it was doctrinally invalid is beyond me.