SPOILERS! TOO MANY TO USE THE TAG!
In X, Seishiro and Subaru fight at the Rainbow Bridge, and Subaru kills Seishiro. Seishiro also says something to Subaru that makes the Sumeragi cry over his death...(going off an amv here) so I wonder whether Seishiro loved Subaru in his own way.
I think that this is true with Syaoran. Seishiro does care about him, but allows his ambition for power to take priority. As for the vampire twins....in Tsubasa, I think that they cursed Seishiro with immortality or something like that. So now he has to kill them to end his curse. They're changed in Tsubasa,because CLAMP wnated to explore different aspects of each character.
It's like the girl with a dog and the other guy from X who turned up in TRC book 5 as demon hunters....