The surefire way to tell if your female killed your male is to see if he looks really beat up. Even though they peacefully coexisted for a time, what probably happened is one came into breeding condition and the other did not. Because the male is dead, I assume he was not ready to breed - perhaps he was younger than the female or not in as good of condition. Your female probably killed him because he wouldn't leave her territory. In the wild, the female would have chased the male away so another male who WAS ready to breed could come but in your tank, he had nowhere to go so she just wailed on him until he died. This is the reason why a male and female cannot be kept in the same tank, especially a small one. The only time they CAN be kept together is in a very large tank with many females per one male and lots and lots of hiding spots for the females to retreat to. More females means no one female gets picked on to the death. This setup can be done but unless you are trying your hand at "natural" breeding, it is not recommended ever.
Keep an eye on your female to make sure she is not exhibiting any strange behavior and keep the water very clean. And you might want to pick up a new testing kit.