#I think I would have risen similar concerns if I was in Thunder's position

[bonefall]

why would righteous thunderstar say "sure moth flight, we'll help you defend against my insane father who's trying to steal your children but only if you give Me one of your children"

Years ago, Sunlit Frost nearly died, receiving permanent nerve damage, from having an untreated burn that went septic. During the First Battle, his good paw was injured and he was permanently left unable to build, hunt, or fight anymore.

If, back then, Sky's Clan had a medic, Sunlit Frost wouldn't have chronic pain to this very day. Gray Wing's breath rattles within his lungs, growing worse by the day. Battles are still constant. Thunderstar knows the importance of having someone who is capable of healing around; and Dapple is now dead without a successor.

Moth Flight and her children are the only cats left with the ancient knowledge of healing that came from their Tribe ancestors.

In his mind, he isn't being cruel. He's negotiating. He is giving them a choice, and why would he risk his cats receiving injuries defending against Skystar, only for them to die of infection as a reward?

He needs one too. Perhaps Spider Flight realized this, and that's why he chose to come to ThunderClan after seeing the writing on the wall.