Female Hamster Killed Sister

Question
My bf and I bought two female hamsters from a local pet shop less than two weeks ago. According to the person who assisted us in the the shop, these hamsters had just been weaned from the mother, a Golden.  They were very small, maybe two inches long or so and nearly pure black (but supposedly Golden variety).  We had them housed in a 20 gal. acquarium outfitted with all the right bedding and toys, litter box and separate sleeping and eating areas, etc.  I bought two diff. recommended books and so far in the short time since we bought them, tried to do all the right things.  They seemed to get along most of the time, but we witnessed them tussling with each other a few times.  Sometimes they slept curled up together, and sometimes they would sleep separately.  One was more active than the other, but I understand that that is not uncommon. Well, yesterday morning, my bf discovered that one of the hamsters had killed the other one, and had eaten about half of her head off (I won't describe it, but I can tell you that I was horrified, and I didn't actually see it myself.) My bf disposed of the deceased hamster, but now we are a bit disturbed and baffled.  Why would a female kill her own sister - and at such an early age  - in such a short time together?  Why would the pet store sell us the two together if this is common behavior? Plus: in the store, there were probably 15 or more hamsters of the same sex in the enclosures for display, and none of them were fighting or seemed injured, etc.  Needless to say, now I feel weirded out by the remaining hamster.  I wonder if she's normal, safe, healthy, etc.  I had hamsters as a kid and never saw this before, but my bf says the very same thing happened to him once with a pair that had been together over a year.  I have so many lingering questions... please give me your thoughts on this disturbing turn of events.  Thank you for your time!!

Answer
One of the many things people who care for more then one hamster face is this. The black hamsters sound like they are indeed golden, they don't have to be gold colored, but the majority are. The more common name is Syrian, although alot of people tack on "Golden syrian" or just "golden" in your case. One thing to learn is never trust what the pet store has to say. Most of the people are NOT very informed about the animals behavior, so they have no idea how to keep them. Most likely the person at the store saw the hamsters being kept together, and figured you could just do that anyway. You can NEVER keep 2 Syrians together, you can only keep dwarf hamsters in pairs like that, another thing pet stores will rarely inform you of. I too would think because they were young, they could be kept together, at least untill they got older, but there's still that risk, and you encountered it. I think the reason that the 2 sisters faught at your house, and not in the store, is because they were most likely born or raised in those cages at the store.  When they moved to your house, it was much smaller then the store display, and it was an unfamiliar enviroment, so this may have caused competitve behavior, sister or no sister. Hamsters can be strange like that somtimes and spontaniously do things, just like we do. I hope I've helped you! :)