Basically, pure evil is when a villain is fra beyond redemption and are completely unforgivable. You can know by checking TV Tropes or the category list of the page
Ratigan did try to trigger me that he is a complete monster. I think he is heinous enough. So yes.
And genocide is truly a heighnous crime
But he tries to kill Hiccup, and there are some villains who qualified for pure evil without killing anyone, which some actually didn't want to, like the coachman.
One question, how much upvotes are needed.
I truly find him evil enough to qualify. The latter never has a chance to be in the good side.