User blog comment:Xandermcc/Arrow and the Flash copying each other?/@comment-24152851-20170505023712

Season 2's main villain is a evil version of the hero (going back to their origins), but completely dominates them in terms of physical power.

Season 2's main villain kills the hero's remaining parent in front of them.

Season 3's main villain is someone who is really feared and famous in-universe and has overwhelming power over the hero.

Hero's world goes down a dark path in Season 3 and hero ends up breaking.

Season 3 makes it look as though hero has/will turn evil (Oliver seemingly became Al Sah-him, Barry is destined to become Savitar)