Cupped and isolated bettas taken from sororities will become aggressive and often vicious. Even the once docile females will become aggressive. So in terms of reducing aggression, you are doing more harm than good.
It would be more affective if you cupped but float the said female. If she is constantly surrounded by other females without any aggression (she should calm down after a while) she should feel "safe" and wouldn't be so vicious. But once she is released, there will still be aggression but hopefully not too vicious.
Aggression to a point of killing in a 20g . . . might be caused by a number of reasons; the individual murderer is by nature vicious, not enough food (females can eat much more than males), not enough hiding places for others to run off to.
Since they are with live bearers, I wouldn't quickly assume a betta initiated the attack. Live bearers (specially platties) are known to nibble on their neighbors. Bettas "attacked" in this way (hit and run) will easily stress which is then sensed by others (all fish in the tank) and will cause more attacks. Try removing the platies and see if you still have deaths.
It's been quite a while and lots have changed since the mid 70's. Back then or IME blue and red may be aggressive but have lower mentality compared to traditional black or green/turquoise mix (wild or traditional colors) - meaning they wont fight for too long. Most bettas that are over aggressive will stress faster and once stressed will take longer time to regain "self esteem". But these bettas can also be very vicious and will readily kill others that are less aggressive. Perhaps their instincts tells them to eliminate possible threats before they are challenged. . . . IDK