It probably died due to something already wrong or due to how it was captured.
Heck, I try to watch a fish in the store for two weeks before I buy it if possible.
Honestly, if it was a new acquisition for the store, it may have been destined to die regardless. Sadly that is often the reality.
You should check with the store and see how long they had the tang prior to your purchasing it.
If I spot a nice fish in the store, I go back every couple of days for a week or so before committing to a purchase. If I miss out on it, oh well, there will be more.