Washington admitted to doing it during the seven years war in 1760. Go rewatch the scene. Search the databases and the wikia as well. Can't blame the templars for everything, buddy.

You need it to be "unclear" because of how obviously nonsensical Connor's reaction is.

I've already said several times here why Connor should be tougher on Washington. Not just revenge, but I talked about his original goal of protecting his village and how that goal is endangered by the discovered knowledge in 1778 that the revolution is HOSTILE to his people and thus is as much a danger as the templars are to Connor reaching that goal of protection. So, you're missing the point, but I've said enough about that.