Downplayed: His nature as an embodiment of elemental Evil is only hinted at; Jason only allegorically represents Evil in the story.
The setting has emotions and good and evil as tangible, quasi sentient forces; Jason just happens to be the one for Evil.
Jason isn't actually pure evil, but rather a manifestation of things associated with it like killing, greed, and theft. It just happens that evil is the easiest thing to call him despite not being 100% accurate.
The main villain Jason is actually Made of... Good?! Turns out Good Is Not Nice.