The most evil person....

[Tar-Alcarin]

Maeglin, is much more suitable for the title, having fallen incestuously in love with his cousin, having betrayed the city of Gondolin to the Dark Lord Morgoth, and having attempted to murder his nephew Earendil.

Aman to that. Maeglin is the most evil elf, he gave the position of his own city away to morgoth, because of that countless noldorians died and fell in love with his cousin! HOw evil is that. Now Eol granted did kill his wife, but that is nothing compared to the pain and suffering Maeglin caused.
And as for most evil Man: That goes to Ulfang the Eastering. Becuase of him the Unnumbered Tears was lost. He turned against his human and elvish brethrin and slaughter them all.