Just because he's a human supremicist shouldn't mean that he can't be enamored of toads, does it? It's not like he wants to eliminate all other forms of life besides mankind. His take on human supremacy is really just about being a racist. He hates demihumans and other human-like-but-not-fully-human creatures.
Besides, toads are just mindless, fun, warty little guys. No threat to human domination over the world. I expect that his personal regard for these little guys also extends to "toadlike" creatures, such as bullywugs (in a weird kind of way- but let's take note, he's not the most stable of personalities). Yes, they're humanoid, but perhaps in his mania, Wastri only sees them as really big toads that walk and talk (kind of).
Or, you could always look at it in the way Trollwad presented it, which is the whole "human supremacy" angle is just a ruse. Should work either way.
