Extreme Betta Killer?!

Question
So I have a betta fish in a 10 gallon tank. At first, I had been told that Mollies and platys were fish that bettas get along with, so I put four (two Mollies two platys) in with him. They surprisingly got along pretty well. Every once in awhile, the betta would peck at the platys fins, but that was it. Nothing too severe. Until one day, when I came home and found one of the platys dead with his fins ripped up. So I guess I learned my lesson, and I bought a tank divider to separate the other fish from the betta. Everything was perfect, until today: when I came home, I found one of the Mollies dead! All the fish are very healthy; I know that for sure. I feed them varieties of food, the temperature of the water is perfect, and the filter is running properly. The only thing odd I noticed was that the dead molly was lying against the tank divider... so I can't help but think that could it be possible the betta fish killed him through the tank divider? I cannot think of any other reasons why he would have died. Thanks for anything you can tell me!

Answer
Mandy,

Sorry to hear you are having problems. Some people say they can put male Beta's with other fish, but I am one that does not agree. I have 11 adults and everyone is in their own tank. I am not sure what kind of divider you have, but I don't think that the Betta got a hold of that one, at this time. Is there a chance that the Betta attacked that one too and you just didn't notice an injury? That is the only thing I can think of since everything, like the water and filter, were doing their job. What are your water readings? Does your divider have small holes in it that allows the water to filter through? Sometimes our fish die and we don't know why. This could be the case. You have stumped the expert. This is the first time I have heard of anything like it. How long ago did the first fish die? I would have to say either it was injured or sick and just died, and not a new attack from the Betta. Let me know as soon as you can the answers to my questions. I am very interested on how and why the Molly died.