To cross the line one must have it on the first place. Only people from the normal families (i.e. families where morals are not 'morals', but physiology - unspoken, unquestioned rules) have the lines to cross. Unfortunately, none of the main SGA characters I've seen so far were from a normal families.
One doesn't need to give an Oath to break it. Oath is already there when one was born, if he was born in a normal family. And that Oath stays there, doesn't matter if you're a doctor, or a scientist, or a military. It's there... Or it's not - and never was.