IIRC from other biographies, being pilloried to his face in a session of Parliament because he received and secretly shared another man’s correspondence was a conversion event that turned him from British subject to fully American. He knew we would always be considered and treated as inferiors to the lowliest Englishman. Australians still deal with that attitude.
The letters were from Thomas Hutchinson, at the time the letters were written, royal governor of Massachusetts, long time foil of James Otis and Sam Adams in run up to Revolution. The recipient had died and the letters fell into someone else’s hands. That third party apparently allowed them to slip into Franklin’s hands, then in London as agent for Massachusetts. Although Hutchinson, an American, loved America, he was a loyalist to the end, and could see the Boston scene pushing toward independence, though it was not spoken of. One of the letters included the phrase “ abridgement of British liberties;” when Franklin sent the letters back to Boston, under strict instructions not to divulge them, he was breaking every protocol, especially as he was being paid 3000 pounds/ year as royal postmaster. The letters leaked out in Boston, were widely publicized , and cooked Hutchinson’s goose for the last time ( a mob had descended on his home and destroyed it some years before). Although it wasn’t known who had actually released the letters, Franklin owned up to it when a duel was fought when an innocent party was accused. The outcome of the duel was not fatal, so they were going to do another go around, when Franklin stepped forward. They hauled him before the Privy Council and he was denounced in a savage manner by the solicitor. Nonetheless, he continued to hold out hope there could be a reconciliation. Sam Adams was instrumental in seeing that didn’t happen.