AIways posted...But there was no reason to believe he was going insane.
Paraphrased:
Citra: What do you want?
Jason: I want to save my friends.
Citra: No. What do you want?
Jason: I want to kill Vaas and Hoyt.
At one point, Jason believes Riley is dead. He could've just waited for the boat to be fixed and left the island, but he chose to go back in and kill Vaas and Hoyt. After he kills Vaas, he chooses to stay on the island, even when his friends do fix the boat. These are not the actions of a sane individual.
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By Evolution, I mean Evolution. As in "I look different from my parents because of evolution" ~OrangeWizard