Nah, he's actually a pretty complex character.
He acts like a normal obedient dude, then goes chuuni after you discover all the awful things he did out of necessity in the war (which was dumbed down in FGO and painted Karna as more innocent than he actually was). He basically gives a name to his sins as a means of trying to deny that he is a human capable of both good and evil, not helped by his severe PTSD.
But once you help him defeat his inner demons and help him accept himself as a flawed human being he confesses his undying loyalty to you. He's a wonderful character.