Most Evil Kaiju

[LSD Jellyfish]

Grand King Ghidorah. I feel bad for SpaceGodzilla and Destroyah. To an extent they just feel like mutants trying to exist. Yeah they cause pain. But they also exist in a world with relatively no concepts of good or evil, sans maybe some stuff said by the cosmos and from Mothra. The same logic goes for a lot of other monsters, they're either mutants just trying to exist (Hedorah), not in control of their actions (Titanosaurus, other incarnations of Ghidorah).

The Mothra trilogy has a better sense of "Good versus evil", and "the earth vs whatever". Both Desghidorah and Daghara exist thanks to human interference with nature (yes Death Ghidorah existed before but deforestation released him). King Ghidorah is just completely alien, and eats children. It goes from planet to planet consuming everything and isn't part of nature or some painful anomaly. It's evil in not just its actions but what It represents in the series.

[Terasawa]

One of the biggest problems with the first ROM movie is that Desghidorah is described and treated as an evil monster but in reality he's just a natural creature that came to earth looking for a meal. The filmmakers didn't even understand their own message.

[Destorogoji]

ROM Ghidorah, Anime Ghidorah, OG MechaGodzilla.

I discard both Destoroyah and SpaceGodzilla for being practically newborn creatures in a completely new environment. Can you point at the kid drowning ants and call him an evil bastard?
I also want to discard GMK Goji, mostly because, even if his actions were evil, his motivation was more like divine punishment, he was the revenge tool of the ghosts from WWII.