I almost beat some False Gods with this card. But unfortunately, Rainbow out-controlled me, and Gemini copied and momentum'd my gigantic Vultures and killed me. Sigh... But yeah, a Dark/Death or Death/Fire deck could probably beat some Gods.
On another note... It might be better to use Mark of Air and Thunderstorms instead of Plagues, because Thunderstorms don't cost much and they kill immediately.
But still, this card is incredibly overpowered. How exactly do you counter it? It's instant kill, and it fills up your slots. Unless you have a couple of Otyughs, Fallen Elves, or Owl's Eye, your space will be filled. Rainbow Godkillers may be able to barely counter Carcinogen, but Mono decks will be absolutely slaughtered.