When Bogardan Hellkite enters the battlefield, it deals 5 damage divided as you choose among any number of targets.
You choose how many targets Bogardan Hellkite’s triggered ability has and how the damage is divided as you put the ability onto the stack. Each target must receive at least 1 damage.
If some of the creatures or players are illegal targets as Bogardan Hellkite’s triggered ability tries to resolve, the original division of damage still applies and the damage that would have been dealt to the illegal targets is lost. It won’t be dealt instead to a legal target.