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.
Again, I will state this fact – which was my only contention. “Evilest” is a word. I was correcting the poster who said, in obvious error, that it wasn’t. I never said it was an eloquent word or that it wasn’t awkward, in fact I said I would use “most evil” myself. Usage of the word evilest can certainly be considered to reflect a bumpkinesque vernacular, though bumpkinesque is not really a word.
But “evilest” IS a word. This is a demonstrable, inarguable fact. It is in the Mirriam-Webster dictionary. Prior to the application of any multipliers, the word will get you a score of 10 in Scrabble.
I never asserted any positive attributes to the word or its usage, only confirmed its existence as a real actual word in the English language. My correction was correct. If you have any other opinions as to its status of the word, you can take them up with the editors of the Mirriam-Webster dictionary, but since this is sort of their area of expertise, I doubt you will win the point with them either.
I have little doubt that you can also find the word in the OED, but you have to pay money to access it online and I do not have a printed version within arm's reach.