I'm very sorry that you weren't told about the wobble beforehand.
A lot of people will claim that a bad wobble has no ill effects on the snake and doesn't affect quality of life, but honestly I'm of the opinion that it does. It's a touchy subject, since the animals can't tell us themselves how it makes them feel. In any case, I think some people can excuse a lot of things in breeding when the animal has a desirable phenotype despite its health issues.
Whether you put it down is really an ethics thing that you will have to work through.
Did you tell the person who sold you it that you were unaware of the genetic issues present in the spider gene and that you would not have wanted to buy that animal had you known? Even if you're still going to keep this BP, I think you should let the seller know so that they will be more responsible next time and put a 'disclaimer' if you will on all animals they sell that are carrying genetic defects.