SableWyvern
Cruel Despot
Do you want the players to avoid killing all the kobolds because you simply don't want to participate in a game where the PCs engage in bloodthirsty revenge?I absolutely love the roleplay and want to lean into it, even though this is not part of the starter adventure.
At the same time, I want to avoid the group later attacking all the kobolds at the Observatory out of pure revenge. One possible condition I’m considering is: Myla’s wings can only be healed if Mek and Minn are not killed.
If this is the case, instead of looking for an in-game way to manipulate the players, I would advise that you just state this up front, so that everyone is on the same page about what they enjoy about the game and what people's limits are. If you want to also provide an in-game rationalistion, feel free, but setting and maintaining shared expectations is critical to ensure this type of thing doesn't just keep appearing. It also gets around potential problems if your players decided to ignore the in-game rationalisation, which could just make things worse if the PCs are angry about having their revenge thwarted and start to focus on other ways to mete out justice.
If the reason you want to avoid it is because it will derail the adventure in some way, I'd recommend that instead of trying to thwart it, identify what the consequences will be and be ready for them. This is actually important even if you do decide to give them in-game reasons not to go on a killing spree, because you'll be prepared if they ignore your efforts and things are derailed anyway.