Character development

[Dr. Briggs]

You know, even in the old ones Jason kind of had some character development throughout the films; he was portrayed as more of an ignorant child in part II, who only learned of killing with his mother's death, but that changed and he became a sort of psychopathic hillbilly with III (For one thing, we never see him "defend" his mother after part II, and many times he kills people who aren't even near the camp starting I think with the store-owning couple from the start of III).

By IV Jason has pretty much changed into a death machine, and in VI onward he's a full-fledged "boogeyman" (His heart is shown as something pure evil in JGTH, which wasn't exactly the vibe we got before IV >.>).