How would you interpret ‘gain the better of him’ in the context?
He was given to a secretive temperament, and it gained the better of him when he fled Mexico. He stopped keeping his journals and became hopeless of the written word and its consequence. He told me that, later on. Every scrap of his writings lost, things he’d kept track of since boyhood. He let go the hope of becoming a man of letters. I can attest. We were acquainted at that time, and if pressed to say what this young man might become, I’d think first of the kitchen, or any profession that suits one who keeps to himself. But a well-known writer of books? No. He read them. But most did, in those days.

How would you interpret ‘gain the better of him’ in the first sentence?
Thanks!
It’s from the Lacuna by Kingsolver.

Question

How would you interpret ‘gain the better of him’ in the context?
He was given to a secretive temperament, and it gained the better of him when he fled Mexico. He stopped keeping his journals and became hopeless of the written word and its consequence. He told me that, later on. Every scrap of his writings lost, things he’d kept track of since boyhood. He let go the hope of becoming a man of letters. I can attest. We were acquainted at that time, and if pressed to say what this young man might become, I’d think first of the kitchen, or any profession that suits one who keeps to himself. But a well-known writer of books? No. He read them. But most did, in those days.

How would you interpret ‘gain the better of him’ in the first sentence?
Thanks!
It’s from the Lacuna by Kingsolver.

Answer

First off, "gained the better of" is not the most common form of this expression. The more common modern expression is "get the better of". It means "to overwhelm or overcome", "to gain an advantage over", or "to defeat in a competition". If a feeling gets the better of you, you cannot stop yourself from doing something, despite knowing that what you are doing is wrong.
In this context, it means that his want to be "secretive temperament" overwhelmed him. It forced him to stop keeping journals and get rid of everything he had written.
Some examples:
"Her curiosity got the better of her and she opened the door and looked inside." Her curiosity forces her to open the door. It's implied that she shouldn't do this.
"He fought fiercely, but his opponent easily got the better of him." Despite fighting hard, his opponent easily defeats him.