Why did your female Betta fish kill male Betta?

It's not typical for female Betta fish to kill male Betta fish. In fact, male Bettas are typically more aggressive than females and may be the ones to initiate fights. Betta fish are known to be territorial, and if two males are kept in the same tank, they may fight until one is killed. However, female Bettas are usually not aggressive towards other fish and are unlikely to kill a male Betta.