Betta fish problem - female fighting with male?

[Gin]

My female betta is very agressive, she has already killed a male. Following his death, she was kept on her own with a more quiet female. We recently bought a new male. The female started to fight with the male straightaway. We separated the male and the females, then a few days later, we re-introduced the quiet female in the male's tank. The male fought with her, so we put her back with the other female, who also fought with her. The poor female is now dead. I tried re-introducing the first (agressive) female with the male, and she has ripped to shreds half of his fins. I don't know what to do, this female is terrible, and I thought if anything, it should have been the male fighting with the girls (as it was the case with our previous bettas). I had thought our previous male had died of fin rot, but I now understand it was our female fighting with him. I don't want to keep them isolated from each other all the time, and I had thought that if the male was left on his own in the big tank for a bit, he would establish his territory. He seems to be more confident (he flares) but it looks like it is not stopping the female from attacking him. What should I do? Get rid of the female? Keeping them separated for longer?