Hard to understand Tulkinghorn's motive.
Why was he so bent on bringing Lady Dedlock down, or humiliating her ? Okay Lady Dedlock disliked Tulkinghorn and didn't try to hide it from him but Tulkinghorn was the kind of man who made three enemies before breakfast. What would he care if Lady Dedlock didn't like him ?
There didn't seem to be any sexual angle either so nothing doing there. And Tulkinghorn's line that he was protecting the illustrious family name and family credit of the House of Dedlock was rubbish as well. Sir Leicester had no children and didn't seem to have any relatives either so after his death it would have been all up with the family credit anyway.