Oh good lord.

This is beyond disgraceful. I really suggest you call your local HS and report this store for neglect/cruelty, which applies even to fish. I've done this in the past, and the store received a warning. Not totally satisfactory, but better than nothing.

For humane euthanization, this link describes how to do it, in detail.
http://www.wisegeek.com/what-is-the-most-humane-way-to-euthanize-a-fish.htm

I hope the other little one recovers!