Why had they chosen him to bear the brunt of their manifold crimes?
That, too, was clear to him. With him in the penitentiary or gone to the
gallows the whole episode would be closed, the men who had put through
the monumental scheme would be in a position to enjoy their boldly
acquired profits with no fear of the law so much as searching for them.
It was necessary to them that some man suffer for their wrong doing.
Now: why Buck Thornton in particular? The reasons were forthcoming,
logical and in order: he was a man whom Pollard hated; already Pollard
regretted having sold the Poison Hole ranch for twenty thousand dollars;
he wanted it back; Thornton happened to be a new man in the country and
new men are always open to suspicion; he happened to be close enough to
Ben Broderick in size and form and carriage to make the deception easy.
And, thought Thornton, there was one other reason:

The undertaking of these men had already grown too big, the work too
extensive, for it to be handled by two men alone. They had confederates;
that was inevitable. Blackie, the saloon keeper in Dry Town, was one of
them, he felt sure. The Bedloe boys, always ready for deeds of wildness
and lawlessness, were others. The Bedloe boys hated him as keenly as did
Pollard, and they were not the kind to miss an opportunity like this to
"break even" with him. It was noteworthy that he had had no trouble with
them since he had shot the Kid's revolvers out of his hands at John
Smith's place last winter; they had left him entirely unmolested; the
three of them who he knew were fearless and hard and vengeful, had not
sought in any way to punish him. Here was the reason.

He went back to Winifred Waverly's letter. She had ended by saying,

"I know that Henry Pollard suspects me of knowing more than I have