At the End of All Things
In the end, Thranduil’s single-handed stand paid off for his people. "In Greenwood the Silvan Elves remained untroubled," while the other realms of the Rings faded fast (Appendix B, RotK, LotR). Legolas carried a bit of Greenwood south with him, and Ithilien "became once again the fairest country in all the westlands," (Appendix A). If the Silvan elves were "more dangerous and less wise," they also seemed to be handier in the mundane affairs of the time, such as homeland defense and recovery.
In conclusion, I cannot see where Evil!Thranduil comes from, except perhaps from ignorance or willful distortion. While the former is perhaps excusable, the latter has no place in Tolkien fanfiction. Expanding upon the work of another carries with it a responsibility to keep faith with the original, lest the tribute of a "fan" become an insult to the author.
And if one can still argue against everything cited above,
I rest my case.
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The Silmarillion, JRR Tolkien
Morgoth’s Ring, JRR Tolkien
Unfinished Tales, JRR Tolkien
The Hobbit, JRR Tolkien
Lord of the Rings: The Fellowship of the Ring, JRR Tolkien
Lord of the Rings: The Two Towers, JRR Tolkien
Lord of the Rings: The Return of the King, JRR Tolkien
The Anti-Evil!Thranduil Campaign, by Sarcastic Elf
In Defense of the Elvenking, by Karri