Mind you, Edward's reasoning wasn't 'lets oppress the Scots', but rather how to deal with a potentialy dangerous kingdom on his borders. His ideal solution would have been a wedding, but unfortunately, Margaret died.
If Scotland were stronger, it would have done the same, in fact both sides were making inroads against the other for as longas there had been a divide.
As for 'self-rule' the concept, as we know it today, simply did not exist in most of the world.
19th century nationalism has coloured our view of the past almost beyond recognition.