Answer: Tavington killing his wife and son
John Billings is one Benjamin Martin's men, and all he has is his wife and his little boy. Tavington, in rage over Benjamin, tries every possibility to get to Benjamin.
Earlier on in the movie, Tavington burns down the plantation which is owned by Benjamin's sister-in-law who is taking care of Benjamin's children, in hope of getting to the children.
Next, he provokes Benjamin, at Fort Carolina, while Benjamin is under the protection of the white flag which he rides in under, in hopes of Benjamin showing signs of aggression.
Later on, Tavington gets a list of some Benjamin's men and is burning down houses until someone reveals the Militia's hiding place and he attacks, killing whomever resists. This is the breaking point for John Billings, as his wife and son are among the victims.