At the beginning as the silent knight he appeared honourable and noble and I'm sure he didn't buy Manfred's 'scruples' about his marriage.
No sooner does Frederic see Matilda, his noble intentions are forgotten to ally with his foe.

Does the idea of marrying Matilda allow him to so easily forget
his dream in the holy land, the helmet and the lines on the giant sabre ?
Apparently.

Frederic came to rescue Isabella, who shouldn't be in Otanto. Now he's willing to offer her in marriage to a false prince in order to marry fair Matilda !?

A Crusader Knight should be aware of the Will of Heaven.

I feel quite disappointed in Frederic. Mayhap we find one twist more in the plot !

What about 3 drops of blood from Alsonso's brow, trickling down the cheek and congealing on the breat, with a pool of blood near the statute ?
I wonder how dramatic that would look on film.