Killed the fish doesnt mean solve the problem. You need to compare the water which came with it and your house pipe water. Check on the PH and temperature. Normally Betta able to get use to all kind of water. Ph between 6-7.5 is fine for them. But the temp try to maintain around 26-30. Some betta able to survive n grow in temp of 32C.

don't place the betta where lot of human activities. Example on the floor. You can use the Indian Almond leaf to make some black water. It will help to cool down and or reduce the stress of yr fish.