He spent years looking for the Dragon. He was sure it existed. In the countries he visited, the inhabitants would not speak to him of such things. They told him he was crazy. One day he accidentally set fire to a house he was staying in. At least, he believed it was accidental. No one told him the candelabras should not be left unattended...
The fire spread through the night and destroyed half the village, killing many people.
In the morning, the villagers asked, "What happened?"
"It was the Dragon," he lied to them, though he almost convinced himself it was true. "Now will you help me find it and kill it before it destroys more of your villages?"
The villagers' fear turned to anger. "Yes, let's find the Dragon and kill it before it destroys more of our villages."
They never found it, and yet they continued the search. Many of them died scouring the deep mountain caves for the Beast. No more villages were burned by the Dragon in the years that followed though. That was perplexing to a small number of the villagers, but they were largely ignored. Most of the villagers still trembled in fear at the mention of the Dragon and they redoubled their efforts to find and destroy it.
The villagers were so bent on finding the Dragon that they often attacked other villages and burned them to the ground on suspicion those villagers knew the whereabouts of the Dragon but were not telling. A few of the attacked villagers retaliated against the attacking villagers, but not many. Most simply fled.
Eventually they made him King - King Drumpf - because they wanted him to have all the power and resources available to hunt the Dragon. They didn't mind giving King Drumpf their gold for the hunt but they got angry at some of the other villagers who wanted the gold to go to rebuilding roads and helping the hungry and sick.
"There will be plenty of time for that after we kill the Dragon," King Drumpf said.