Poison and Herold

[STS Lincolnite]

Vaughan Shelton, author of Mask For Treason, claimed that David Herold tried to poison John Wilkes Booth. He claimed the field glasses Mary Surratt delivered to John Lloyd contained poison that Herold put into Booth's whiskey. According to Shelton, Booth drank deeply from the bottle of liquor brought to him by Herold at Lloyd's. The slow poisoning of Booth went on for some time.

So did Herold try to poison Lincoln? Booth? Both? IMO, neither story is true, although Herold may have actually delivered a bottle of castor oil to the White House while working at Thompson's drug store.

I am not familiar with this book (perhaps thankfully). What does Shelton state is Herold's motive for poisoning Booth? And what does he use for references to corroborate this theory?