angel capturing the sun

No, Pascal, you should not use more flash. Keyword is 'should'. If you WANT to use more flash, then you should use more flash.

If you are in doubt about whether or not you should use more flash, then I would say that you would learn a lot by using more flash and comparing the two results.

If you shot this photograph for a Fill-Flash assignment, then yes, you should use more flash.

Personally, I feel the message suggested in the title is aptly conveyed by the darkness of the angel. I feel that any more shadow detail would reveal the inanimate nature of the angel (I assume it's a statue), so its present rendition is, in my opinion, effective. Also, the darkness of the angel could symbolize that it's a Dark Angel, one of the Fallen, and so its attempt to capture the sun has different ramifications than if it were a regular angel just out trying to get a little sunshine, so to speak.

It's a very good image, technically well done, full of meaning, and open to multiple interpretations, all of which add to its appeal.