Nefarious

Adjective - (typically of an action or activity) wicked or criminal.
Origin - from the Latin nefarius, from nefas, nefar: wrong (from ne:not and fas:divine law)

Oh no, it's the Archfiend Marmot of Nefariousness!. Ha ha ha, I'm soooo scared. Ow! It just stabbed me in the eye with its nefarious horn.

 

What? Yet another word ending in -ous? Yes, call me wicked, criminal or, better yet, nefarious but I like words ending in -ous. And it's my choice and if you don't like it, I'll slip a scorpion into your breakfast cereal. That's my style of nefariousness.
I think it is the classiness that attracts me to this word: it takes a certain type of criminal to be not merely wicked or nasty. An evil genius would probably fit the bill. And not one that always fails due to overly elaborate schemes a successful evil genius who never gets caught. And with glasses as well. And possibly a mustache, but it's not obligatory.
Also, I think cats are quite nefarious.