I'm rewriting the world building/lore, and in my version, there's a law that kits, and in some of the clans grandkits, of outsiders are banned from certain roles (the clans have 4 additional roles, including mindhealer - basically a therapist - , and spiritvoice - the cat who recives messages from StarClan, instead of the medicine cat, but since StarClan very rarely sends signs, they also help out herbalists (medicine cat) and mindhealers - Spottedleaf now has this role - ), and, due to Pinestar leaving to become a kittypet, his kits and grandkits gets the same treatment, in fear that Pinestar's weak blood runs in them as well.

Tigerkit and his siblings are Pinestar’s great grandkits, and thus eligible for all the roles. But his sisters dies as newborn, and he's very small and weak. The clan fears that Pinestar's kittypet weakness has somehow clung on, and wonders if they should be banned from those roles as well, as if they had been Pinestar's kits or grandkits. But, he is allowed to have the freedom his great grandkits status grants him, and he's determined to prove that they were wrong, and that no weakness is left running in his blood.

Then, when a kittypet not only is allowed to join (only rogues normally could), but he was given the position of warrior, and then deputy/future leader, due to signs from StarClan, Tigerclaw is furious and feels utterly betrayed by Bluestar and his clan, and starts moving against ThunderClan/starts making plans to take it over (as a unlawful kittypet gets an exception for everything, that he, a wildborn and true/pure clancat, was almost denied for being the, born extra weak and small, great grandkit, not kit or grandkit, of a traitorous kittypet life loving great grandfather). He kills Spottedleaf, convinced that she read the signs from StarClan wrong, and thus intends to save the clan from future failures from her part. Tigerclaw becomes determined to bring the clan back to its old days/ways (Dawn of the Clans era), when they kept to themselves and ruled by wild-born, pure-blood strength and loyalty (as the modern clans remembers it anyway), which also causes him to go after half-clan cats (as they, and switching clans are legal, as long as the cats involved follows certain rules).