Congratulations!! You just got a charter membership in the Dunning Kruger club. Hint you don’t get the membership for being wrong you get it for being wrong and insisting you’re right. Look up your neologism in dictionary.com.
It is not a matter of expanding one’s vocabulary; it is a matter of communicating effectively.
Standard English is a combination of grammar rules and common usage. There are many exceptions to the various rules, and these often result from common usage.
Whether or not one can proclaim the word, evilest, as legitimate, its usage conveys the impression of a lack of eloquence, if not actually a lack of literacy (such as, by a non-native speaker/writer).
The nominal rule is that multi-syllabic words use the modifiers more (comparative) or most (superlative); two-syllable words are mixed: The word, happiest, is common usage; the word, evilest, is not.
I consistently scored in the top half of the top one percentile of college-bound students; Standard English is something I know well.
As it happened, I read this thread in the company of a friend who has a degree in English, and who taught English and Literature (especially Shakespeare) for nearly twenty years. She also once, by the bye, coached low-income students, including ESL ones, to a first-place Academic Decathlon upset over the specially-tutored students of favored Palo Alto High School (across the street from Stanford University). She is the most literate person I know.
I asked her if she would ever endorse the use of the word, evilest. She laughed out loud. When I pressed her for her justification, she said it was not a matter of a grammar rule, per se, but one of common usage.
As an author, the only time I would use such a word would be as an element of dialogue, given to an untutored bumpkin.