My real thoughts are that he may have some connection to Uta in his past, but that’s all. :O
As for fun headcanons, let’s go!
– Donato probably broke all his seminary rules.
– When he was really young and all alone on the streets of Russia, he went into confession out of despair.
– The priest who heard his confession took pity on a ghoul, and decided he could raise him as a proper Catholic.
– Unfortunately, Donato was a rowdy youngster a la Uta and Yomo. He put on a good face for his foster father, but his sadistic tendencies began to grow as well. Because in sadism, he found the power he’d always lacked.
– When he was found out, he fled Russia and never saw his Father again. He took in kids initially with good will, but his sadism eventually got the better of him.
– He saw Amon as the kid his Father wished he was, so he spared Amon.
– He’d probably deny all of this.
Mercy and Magic 