Almost any lfs will take in fish for free. Try others in your are if Walmart won't take it back.
You mentioned that you wanted your betta to kill the goldfish. As much as I don't condone this, I can use it to see at least that you don't really want the goldfish. In which case, if you can't rehome her, euthanizing her would be kinder than allowing them both to live (for the shrt time that they would survive) in a 1.5g tank. At the beginning of starting and maintaining... there is a sticky on humane ways to euthanize a fish, should you decide this is for the best.
You could get a arger tank for the goldfish but remember that they need at least 40g to grow out, and larger eventually. I would never recommend keeping both fish togethert since goldfish won't live long in the temperatures that goldfish need, and bettas won't live long in the temperatures that goldfish need.
I would however recommend a tank upgrade to 5g or larger for your betta. 5g is the recommended minimum for bettas, and they are easier to keep than a 1.5g. They are also easier to find heaters and filters for, and have more space to fit them in.