>>827422
>Zouken is evil
That's the kicker: Zouken himself his evil, but his goal itself is not. He only wishes to stay alive so he can fulfill his wish. The problem he has is that in Nasuverse logic, souls have a lifespan, and he happens to be some 500 years old. When he was younger, his wish was to create a utopia: The problem being, he himself was not a good person, and even if his intentions were good, he was screwed up from the start. When he moved to Japan, away from his native lands, his soul deteriorated even faster, and his objective shifted to just staying alive. He seems almost aimless because he has quite literally forgotten what he originally wanted, and devotes all of his energy to staying alive. He wants to create the Grail so he can wish upon the Grail to stay alive long enough to wish upon the Grail.
>Kariya and the grail
Not his brother, his great-(x20)-grandson. He fucked over Kariya because he hated him. Makiri are not supposed to be heroes trying to save people: They're supposed to advance their own goals. He did not expect to participate in the 4th Grail War, because he already assumed it would fail, and he could have Sakura's child (who would be much more powerful than Kariya) do it for him in the 5th War, which at the time was presumed to be another 60 years in the future. Kariya coming in meant nothing, because Kariya was too incompetent to win. So instead, Zouken decided to torture him for being a bad heir.
>Will he bide his time and wait for an opportune moment to kill Shirou or just attack him on sight to his own detriment?
He doesn't want to kill Shirou at first. He doesn't care. In HF, he doesn't decide to kill Shirou until Shinji loses. When Shirou and Saber investigate Ryudou, he damn well nearly does kill him, and actually does in multiple Bad Ends. The only reason Shirou survives is Rider, who Sakura sends in to save him - something Zouken was not actually aware of at that moment. After that encounter, there is no reason to kill Shirou, since he's no longer a Master. If anything, killing him would be the random evil.
Further, from then on, Zouken could have Assassin kill him at any moment. He doesn't, because he wants Shirou to keep alive, since his presence aids in Sakura's development as the Black Grail, which is exactly what Zouken wants to happen.
>Will he manipulate his son and daughter to support his goals or just continually torture them until they're incapable of supporting him?
Shinji is useless except for his penis, which carries Makiri genes. Sakura's only purpose is to take those Makiri genes and give Zouken a proper descendant who is actually tied to the land in Fuyuki, and can become a proper Makiri magus. The kid he wishes to create from them is the only purpose they have. And since Shinji is more than happy to rape Sakura, Zouken already has what he wants.
I definitely agree that Zouken is absolutely over the top in terms of how fucked up he is, but it's not as if he's random about it. It makes perfect sense why he does what he does, in the context of who the character is, and in the context of the story itself. It's just extremely difficult to imagine a person who is actually fucked up enough to go to the lengths that he's willing to go to.