Believe it or not, it is not just dictionary.com that lists "worser" as a legitimate word. One of my hard-copy dictionaries (Random House, c1968) lists it as a dialect word, used as an adjective or an adverb, meaning "worse."
I would not have guessed it, and I only checked it out of curiosity after seeing this question.
This does not make it acceptable in good, standard English. But it is actually a real word in some dialect or other of English. I don't know which one it would be.