Yeah, I noticed that even in SR, it's not like when Katara didn't kill the guy Zuko jumped in and said, "What? You have to kill him!" I definitely think all of Zuko's "You'll have to do it!" is more about sharing Aang's insecurities than really looking down on him for them. Zuko's pretty explicitly struggled with his own lack of killer instinct throughout the show. It's nice that neither he nor Aang (nor Katara) killed the person they had to face down.