Cherry barbs are known to be hardy fish so your water quality must have been really bad. You should keep up with the partial water changes weekly, maybe even more if your not cycled yet. What are your water parameters (ammonia, nitrite, and nitrate) readings? What size tank and other fish?
My Cherries chased each other around all the time. It never led to any deaths and it looked more playful than anything. However, if your female cherry is really sick and dying, they could pick on her because they sense the weakness. It's not just cherries, but a lot of fish will harass fish that are about to die, and then pick and eat them. It's like the nature's law of survival of the fittest.
If you have another tank, you can use it as a quarantine / hospital tank. Otherwise, i don't know what to tell you other than let nature be nature and to be a little more diligent in your tank maintenance to prevent dangerous levels of ammonia and nitrites getting out of control in the future.
