Is there ever acknowledgement of a bad elf, or even an elf doing a very bad thing (because he is bad)? I've heard that bad elves are orcs and good orcs are elves, or something like that.
Similar to this: Were there ever any "good" Orcs?
1. Orcs are not "Bad Elves"
Tolkien never clarified the origin of Orcs, but many theories were proposed. The most accepted one is that Orcs were Elves captured and tortured into a "mockery of life" by Morgoth. Getting twisted to Orcs had little to do with an Elf's moral fiber.
2. All of Eru's creations seem to have the ability to be evil if they choose to
Tolkien's world has Eru, the One God. Eru created the Valar (a pantheon of lesser gods / greater angels), their attendant Maiar (lesser angels), Elves, Men and Dwarves (by adoption). We even have examples of evil Valar (Morgoth), evil Maiar (Sauron and the various Balrogs among others), so even if the texts lacked evil Elves (which they don't), there is no reason intrinsic to the world-building why Elves could not be evil. There is nothing inherently "non-evil" in any of Eru's Children.
3. Evil Elves in canon
By the time of the Hobbit and The Lord of the Rings, very few Elves are left in Middle-Earth - most of them dead or sailed. But the account of earlier times, the Silmarillion gives us accounts of many Elves who did evil.
Feanor: Once the greatest of the Noldor, he forged the first weapons for the Elves, led the first Kinslaying, where he and his followers massacred a whole bunch of defenseless Elves in the land without death just because they wouldn't give him their boats; stole those boats and then abandoned half of his followers to a dangerous and fatal foot journey over ice because he was jealous and suspicious of his half-brother.
Feanor's Seven Sons: They may be said to have started out as 'just following orders' and tried to make things right for some time after Feanor's death, but they ultimately led two more massacres for their father's crusade.
Eol: An Elf who kidnapped and forced his wife to marry him, then when his wife Aredhel successfully managed to take her son and flee, he followed her to try to kill the son (Aredhel fatally intervened)
Maeglin: Eol's son, who fell in love with his cousin Idril (taboo among the Elves), jealously sold out their Kingdom to Morgoth for the promise of the Kingdom and girl. Note that he had been brought up by Idril's father as a prince and had great esteem in the Kingdom already. He was the only Elf in Tolkien's works to willingly and knowingly work for Morgoth.
And these are just the bad eggs of ONE single Elven family - the House of Finwe.
Tolkien's conceit when it comes to Elvish information is that most of what we know is information as known to / written by people affiliated with the House of Finwe. We don't have as extensive information about individuals when it comes to the Sindar or (any information when it comes to) the Nandor, Elves who never sailed off to see the Valar. We also don't get much information when it came to common/non-Royal Elves.
While the political situation of Elves in Middle-Earth (not only did all the Elves we hear about know of the existence of gods, even amongst the Elves who did Evil, the majorly politically significant ones were all opposed to Morgoth) meant that we don't know of any Elves who worshiped Morgoth the way the Black Numenoreans did, we have no canonical reason to believe that individual elves could not be as capable of being evil as individual humans. The Feanorians and Maeglin, all had their followers who followed their leaders in their misdeeds.
One of the characters in the Silmarillion is an elf called Eöl. He (sort of) raped his future wife, then murdered her during an attempt to murder his son. He then cursed his son to die a horrible death.
It has to be said that he didn't really cover himself in glory.
He ensnared Aredhel Ar-Feiniel, the wilful sister of Turgon, when, losing her way, she ventured into his forest. He wedded her, not wholly against her will according to The Silmarillion, but by force according to Quendi and Eldar and without informing her family or going through the customs of the Noldor.They had a son Maeglin. Eöl resented the pride and presumptive authority of the Noldor and, given to secrecy much like Turgon, refused permission for Aredhel and Maeglin to leave Nan Elmoth to seek out their Noldorin kin, the sons of Fëanor. Aredhel and Maeglin later left secretly for Gondolin, stealing Eöl's sword, Anguirel.
Eöl pursued them and entered the Hidden Way of Gondolin, and was captured by the guards and brought before Turgon, the king. He wished to go away with his son back to Nan Elmoth leaving Aredhel behind. He claimed Maeglin saying that as Turgon was Aredhel's brother, she could stay behind. Turgon would not allow Eöl to leave Gondolin, offering only the choice of staying or dying. Eöl would not acknowledge any authority of Turgon over him and refused to remain, choosing instead death for himself and his son, Maeglin. He tried to kill his son, but his javelin hit Aredhel who stepped in front of Maeglin. She called for her brother to spare Eöl, but the javelin was poisonous and she died before she could speak her last words. Turgon decreed Eöl was to be put to death by being thrown from a cliff. Before he died, Eöl called out a curse on his son for betraying him, that Maeglin should suffer the same fate of his father. The fulfilment of the curse is told in the Fall of Gondolin. - Wikipedia
Yes, I'd say the best example is Maeglin, who betrayed the location of Gondolin to Morgoth.
In the distant past, Elves were as prone to folly and evil as Men. By the time of the events in "The Hobbit" and "The Lord of the Rings", however, Elves have got that kind of weakness out of their systems. It's why they're so reluctant to act or advise - after thousands of years they've seen so many well-intentioned acts go horribly wrong (or worse, horribly right) they prefer to meddle as little as they can.
The Silmarillion and other works go into more detail on this than The Hobbit or LOTR. Way more than I can cite here.
Men are more susceptible to evil deeds...partly because they have a more self-serving nature, and partially because they have a shorter lifespan in which to learn from their mistakes (although most fail to learn from the lessons of history within their own short spans). Their have been examples of Elves who have been subject to greed, jealousy, and arrogance (Feanor and his line...perhaps misled by the wiles of Melkor, but they killed their kin for their boats and stained Valinor), as well as some downright nasty ones (Eol, the Dark Elf blacksmith, who possessively wedded Turgon's sister), and their son, who betrayed Onolinde out of jealousy of status.... Even King Thurandil was not kind to the companions of Bilbo Baggins, Ring-Bearer