I need help again
“It’s easy to say that now, but to what purpose? Think about it.” He looked every inch a broken
man, the face scarred, the eyes a dark stain. “No one knew the truth but Julia, and she was dead. Who would believe me if I claimed adultery as my defense? You’ve seen the trials same as me, the
desperate men willing to lie and squirm and barter their souls for the barest chance of a decent
verdict. My testimony would look like a string of self-serving, calculated lies. And what could I
possibly get from it? Not sympathy or dignity or reasonable doubt. I’d open myself to cross examination and look even guiltier by the end of it. No, I stared down that road more than once,
thinking about it. I’d humiliate Catherine and get nothing for it. Julia was dead. Bringing up the
relationship could only hurt me.”
What does "string" mean?
What does "reasonable doubt" mean?
What does "I’d humiliate Catherine and get nothing for it" mean?

Question

I need help again
“It’s easy to say that now, but to what purpose? Think about it.” He looked every inch a broken
man, the face scarred, the eyes a dark stain. “No one knew the truth but Julia, and she was dead. Who would believe me if I claimed adultery as my defense? You’ve seen the trials same as me, the
desperate men willing to lie and squirm and barter their souls for the barest chance of a decent
verdict. My testimony would look like a string of self-serving, calculated lies. And what could I
possibly get from it? Not sympathy or dignity or reasonable doubt. I’d open myself to cross examination and look even guiltier by the end of it. No, I stared down that road more than once,
thinking about it. I’d humiliate Catherine and get nothing for it. Julia was dead. Bringing up the
relationship could only hurt me.”
What does "string" mean? 
What does "reasonable doubt" mean? 
What does "I’d humiliate Catherine and get nothing for it" mean?

Answer

'A string of lies' is a series of lies, one after the other.

'reasonable doubt' is a legal term. In court, the accused has the right of "reasonable doubt'. If there is 'reasonable doubt' he should be found innocent. The speaker by saying 'Not sympathy or dignity or reasonable doubt' is making a symbolic reference to this legal term. He is saying she won't give him the 'right' that perhaps there is doubt of his guilt. ie. That he may be innocent.

"What does "I’d humiliate Catherine and get nothing for it" mean?"

He is saying that giving testimony won't achieve any good. What he would say/reveal would humiliate Catherine and he would gain nothing himself from this testimony.

Answer

1. In this context, a string could be thought of as a series of lies. It would look like he was telling a lot of lies, one after another.
2. Reasonable doubt is a legal term in the USA. In order for a court to find someone guilty of a crime, they must prove that that person is guilty beyond a reasonable doubt. He is saying that if he testified, he would appear guilty.
3. His testimony would be embarrassing for Catherine, and he would gain nothing from testifying. His testimony would hurt her, and not help him at all.