Yeah..males with females aren't really a good idea unless you're breeding. Girls can be just as mean as the boys! You're lucky they got along :) Since he died from unknown causes, I'd strip the tank down and do a good scrub and then put Karen back in so she doesn't sick too.
Have you read about the betta sororities? I have yet to try one, but I'm hoping I will be able to, soon! Since you have a 10gal with just one girl..you could try one. Read about them in the Betta Care section on here, and lemme know what you think :)
Aw :( I'm so sorry to hear that! At least you could give him a nice home for awhile. That was really nice of you to rescue him. Wal-Mart really is bad with their fish care, I wish they would either stop selling them or take better care of them!
Are you going to get another one? If you are, I'd clean out the tank REALLY good just in case there's any germs in there that could infect the new guy. Burning hot water and about 2 cups of white vinegar is how I clean my tanks after one passes away or changes tanks. But be sure you rinse it REALLY, REALLY good with really hot water when it's done soaking. And when you think you're done rinsing, do it again!:) WARNING: Bettas are like chips, you can't just have one! ;]