There have been analyses of this line that ask – is he meaning BREAK it as in break character? And it’s certainly possible – but it seems fairly obvious to me that he means the oath. If, after basically swearing herself into a miserable existence, should she break it, (she does, in fact, break it – as we know she will) then, what? Misery. Obviously.
Oaths are really made to be broken, aren’t they?
And the more fervent the oath, the more likely it will break and the more an audience will want to see it broken. A story where someone swears intense fealty and then just keeps that oath? Well, it’s like putting a gun on stage and never shooting it.