It looks like fin rot or a fungus (columnaris if he died so quickly?). It seems like his anal fin was also being affected, yet not torn or bitten.
For future reference, I would do a 100% water change and add 1 tsp of aquarium salt per gallon of water and do 100% changes for 7-10 days. Discontinue use of the aquarium salt after 7-10 days as well, as it could do him more harm than good over an extended period of time.
What temperature was/is your tank at? It is suggested to have the temperature at 76º-80ºF for keeping bettas.
I am sorry for your loss.