Hiya Boo Boo and welcome to AquariumAdvice.
Capt.Gorilla asks some very important questions. What are the water parameters? What size tank? How long have you had the koi? These questions may help point to a cause of your fish's death.
Hole in the head can occur in goldfish, but its pretty rare. It usually affects cichlids in the freshwater tank. Its quite possible your koi do not have HITH, but another type of infection. Keep a very close eye on your others; if the red areas do not start to reduce soon, its likely they have a bacterial infection and you will need to treat with an antibiotic instead. Kanacyn is one of the more effective meds out there atm. Also, keeping the tank water as clean as possible is a must, or else treating with meds won't be effective as poor water parameters will continue to stress the fish and reduce their immune systems.
aka Cycling Guru and the Ich Slayer
*glares at Terry and QTOFFER*
Card carrying member of FTAS & GCAS.