And yet when he came to suspect Small's villainy, his whole nature revolted at it.

But having broken with Ralph, he thought it best to maintain an attitude of apparent hostility, that he might act as a detective, and, perhaps, save his friend from the mischief that threatened him.

As soon as he heard of Ralph's arrest he determined to make Walter Johnson tell his own secret in court, because he knew that it would be best for Ralph that Walter should tell it.
Bud's telling at second-hand would not be conclusive.

And he sincerely desired to save Walter from prison.

For Walter Johnson was the victim of Dr.Small, or of Dr.Small and such novels as "The Pirate's Bride," "Claude Duval," "The Wild Rover of the West Indies," and the cheap biographies of such men as Murrell.