Wouldn’t the natural disposal of Castamere & Tarbeck Hall, after the extermination of their houses be to give them to Kevan and Tygget or to Tyrion or to Tommen through Cersei, rather than lie around to be given to Spicer who is even lower than an unworthy-of-a-junior-Lannister-husband-Westerling? If the Reynes were the #2 western house, they had to have vassals, so are those paying taxes direct to Casterly Rock & that way Tywin keeps all their revenues, instead of taxing an intermediate lord?

Well, keep in mind that Tyrion doesn’t exist yet when Tywin pulls off the Rains of Castamere….

Kevan and Tygett make sense, although there may have been some pushback that Tywin’s actions were radically skirting the rule of law as-is, so when you’re publicly arguing (to the king) that you’re acting to restore law and order, massacring people and taking their land to give to your younger siblings is not that convincing.

At the same time, Tywin seems to have been very jealous of Casterly Rock’s power and wealth – while Kevan and Tygett were taken care of as befits a Lannister, there’s no cadet branch, no gift of land or a holdfast. Instead, Tywin gives them rewards that keep them under his thumb and retains all the wealth and power for himself.