What's a word for an emotion that's akin to happiness but felt by someone so mean that you doubt they're capable of true happiness? As "smirk" is to "smile", this word is to "happy".
Victor Hugo has often been quoted as using "dark happiness" to describe this sort of thing. I don't believe I've ever personally read the book or piece this was pulled from, though. There is also a psychologist by the name of June Gruber who researches happiness. Though she does cover maladaptive happiness, she never uses a term for it other than the aforementioned, which makes me think one does not exist in English.
You might also want to research the Greek roots that comprise the word epicaricacy.
I'm also reminded of the recent use of "troll" to describe individuals that derive happiness from malice and discord.
I've also heard "feigned happiness" used in a few lectures, but not sure that's mainstream enough yet to be used without clarification.
As @john-lawler mentioned, schadenfreude is also a possibility depending on the flavor you're going for. Avenue Q used this word. If the following conveys what you're going for, then schadenfreude may be the word for you.
DISCLAIMER: Schadenfreude is not a word originally conceived in this musical. Google indicates that this word entered the English language around 1840.
"Schadenfreude" from Avenue Q, written by Robert Lopez and Jeff Marx. Great musical if you've never seen it ;-)
Perhaps malicious merriment?
(Not a single word, but other than the Germanic, we are at a loss so far.)
I would suggest something from the latin word risus.
See for instance risible per merriam-webster:
This root is often associated with mocking laughter and smiles rather than the more normal kind.