Providing that as an option could potentially be interesting, but I think many of our users would be very upset with that as a default behavior. We realize that sometimes people might be a bit late with their payments, and destroying an active droplet could mean taking down someone's business. That's not something we want to do lightly.
In the mean time, you might be interested in the ability to set up billing alerts to notify you if you pass a certain billing threshold. Check out the blog post introducing the feature:
Thanks for the feedback!