Why wouldn't Seth take Kate with him to El Rey?
El Rey is not the Eden it is purported to be. To stay there, you have to deposit your money in the town bank, with outrageous fees that get even more outrageous if your balance falls below a certain amount. Only first-class luxuries are provided, and you pay for them, and people end up doing without or wearing their clothes till they're worn to nothing to avoid paying for more. If you cause trouble, you're a victim of "suicide" or an "accident," and there's no social life because everyone there is suspicious and paranoid of each other. When your money runs out, you're put to work as a street sweeper or something else and have to scrounge for your food. When you're no good for that, you're killed and eaten. Whether or not Seth knew this (it's said that no one ever escapes from El Rey, so no one outside El Rey knows the truth) is unknown. So, whatever his reason for refusing to take Kate with him, Seth actually did her an enormous favor by leaving her behind with a large wad of money.
A Note Regarding Spoilers
The following FAQ entries may contain spoilers. Only the biggest ones (if any) will be covered with spoiler tags. Spoiler tags have been used sparingly in order to make the page more readable.
As the sun rises, only Seth and Kate are left standing, still fighting off the remaining vampires. Suddenly, Carlos knocks at the door, and Seth yells at him to kick tt in. The intruding sun kills off the rest of the vampires, allowing Seth and Kate to escape into the sunlight. Based on the fact that Carlos picked the Titty Twister and got Richie and the Fuller family killed, Seth bargains with Carlos for a lower rate. Kate asks to go with him to El Rey, but Seth refuses and gives her a handful of money.instead. In the final scene, Kate drives away in the RV, and a long shot reveals that the Titty Twister is simply the top tier of a buried Aztec temple with piles of trucks and bikes from past victims dumped behind it.