OK here is where u failed.
1. You put in HOT water... did you messure the temperature at all in the water?
2. You Changed the entire water.... the only time this is safe is if you got it from a straight RO/DI tap, where the TDS is 0... dont know what that means? it means u shouldnt do a 100% water change until you do.
3. You added Salt... Betas are mostly fresh water fish. You also most likely brackished the water unless u had a hydrometer, which i don't think you do, as mostly salt water reef tank people use those, and so my deduction poor watson is:
The fish died because u cooked it in water too high for tolerance, on top, it was laced with copper and chlorine coming from tap, which you also made brackish.
In short, you literally made fish soup out of the poor thing.
Betas are one of the easiest fishes to own since they are labyrinth fish, meaning they can take air for some periods of time, and require air.
Betas also are not very picky about water quality until it becomes full of there feces and it smells like a algae infested lake.
Betas still however require you to do some research on how to keep them alive, as they are not pixils in a VR world, where you cal roll back your data to the last save point.
My advice, there are TONS of info on how to keep a beta alive.
Start reading, compare and see what is contrasting, and go with whatever more sites seem to recommend, because the internet also has a lot of trolls which would love to see you grief as they gave you advice to kill your fish.
http://www.reefcentral.com/forums/showpost.php?p=8818063&postcount=145
Do what this guy did and have CATFISH...