Was the Hospitaller considered an angel in Kingdom of Heaven?

Yes, the Hospitaller (played by David Thewlis) in Kingdom of Heaven is heavily implied to be an angel or a divine messenger. Writer William Monahan confirmed in the DVD commentary that the character is meant to represent an angel. 
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Key details supporting this interpretation include:
Supernatural Elements: The character appears and disappears mysteriously in the desert without a horse.
"Angelness" in Action: He tells Balian he has "not heard" that Balian is outside of God's grace, implying direct communion with God.
Guiding Role: He acts as a spiritual guide for Balian, offering wisdom, appearing to know more than a mortal, and emphasizing mercy and duty over the fanaticism of the Templars.
The Director's Cut: The Director's Cut makes the implication more explicit compared to the theatrical release. 
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While some viewers might interpret him simply as a very pious, observant knight, the filmmakers intended him to be an angelic figure ensuring Balian follows his destiny.