Kikuru would have been fully justified to just stop outside of their range and headshot Saran.
Nah, their law states that any Guard killing a person gets the death sentence.
And the police has magic to determine the ground truth after-the-fact.
As Kikuru arrives at the shelter, taking his bow and shooting Zuzo who is currently using his hair magic to rape Noma, or the other guy pinning down Meidena and threatening to rape her could be justified as self-defense. Especially directly after rescuing the back-stabbed Hanabata. But Saran is just standing in the back smugly observing his teammates actions, which is hardly an action that could justify going for the sneaky kill with the bow from afar. Shooting him would not solve the immediate threats Meidena and Noma face; it would even be counter-productive as their leader's death could provoke more rash actions towards Kikuru's team.
Beating the whole team Saran to pulp, letting them suffer for a while, and then handing them to the authorities is the much safer route to take