Why does King care at all if Starr testifies? The worst she did (or could do) was point out that the King Lords ruled the neighborhood, it seems to me, which the police gang unit would have already known. There never would have been any reason to name King himself (or whatever his Christian name is).
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King cares so much whether Starr told or not because he is a drug dealer and he runs the king lords which are a bunch of drug dealers. Therefore they would go to jail if they are caught selling drugs and when Starr testifies, she says that Khalil was selling drugs for the king lords because his grandmother was sick with cancer. So she told not just the police in garden heights but everyone that King was the leader of the king lords because the police hadn't known who it was before she had testified. (Dont worry it's a good question. Also you should read the book it's really good and goes more in detail on what had happened)