It is complex, I agree. Why I rarely give my word on anything, because it does count and it does get kept, despite everything.
In the case of the extreme of giving your word to a leader who eventually becomes a monster - who have you given your word to and why? Have you pledged to the person or the ideal for which they stand? A word goes both ways, remember. If they break it, you can, with much less stress on your conscience, be considered free to act as you will.