Even KQ8 is relatively lighthearted compared to other games in the series, and in comparison to other game series. Roberta placed her kind of inherent sillyness or whimsy into most of the good characters an even into some of the lesser villains. Some argue that KQ3 is the darkest in someways. Even KQ4 in some ways.
Although I'd argue it is the fan games try to make the series dark and "gritty".
.