I have found that almost 100% of players kills Dragon Hunter. Why?
If you kill him nobody will slay the dragon and he will attack you!.
It's not necessary to kill him to go further :) The side quests may have far-reaching consequences. Sometimes you will get it after the 4 or even 5 missions.
Side quests are not a random or formal mission. Consequences depend on your action and may vary depending on your deck, active warlord and so on. I keep adding new sides quests in the game.
Don't forget that you can use Amnesia button to start the campaign from the very beginning ;)