To vampires, blood is like alcohol; some drink occasionally while others are raging hemoholics. Animal blood would be like a beer while human blood would be like a hard liquor. You are a vampire trying to explain to a human why Count Dracula is a bad representative of your species.
Why would Malcolm take Jem and Tessa's word when they weren't even born when Annabelle is "taken from him"? Blaming Magnus makes more sense, since Magnus was alive and most likely knew if Malcolm and Annabelle. Also, how does Jem not know Annabelle isn't an Iron Sister? Or is becoming a Iron Sister like becoming a Silent Brother where they choose another name? Like how Jem is/was Brother Zachariah.