First, keep in mind that this isn't a sprawling underground city -- it's a single goddamned room, and most of the room is lava. And Dodongo is not a lava monster -- you kill him by throwing him into it. So this beast exists entirely on a small stone ring barely wide enough for his own body, encircling a remorseless pool of magma.
"You know, man ... you can just have the artifact. In fact, here, take some of mine."
He can't even turn around without risking tumbling into the pit of eternal hellfire. And most importantly, there is no possible way for King Dodongo to ever get out of this cave. Link falls in through a tiny hatch in the ceiling, and escapes through a magic portal -- there are no exits. So Dodongo wasn't terrorizing the countryside or building a moon laser. As another character tells you, he's only there to eat whatever the cave has to offer. Meaning rocks.
That's right -- rocks. Not virgins or children, but rocks. Motherless deposits of sediment whose disappearance from the earth results in the shedding of precisely zero tears. King Dodongo lives in a hole in the ground, eating rocks and minding his own business. And Link breaks into his house and throws him into the lava, for the express purpose of proving to the townspeople outside that he is a real man.
If there's more to manhood than destroying things, we don't want to hear about it.