She seems less evil and more just trying to free Edenia. Whether or not she would have any more selfish motives than that obviously we don't know but she is seemingly less evil this go around.

She obviously wants to RULE (key word) Edenia.
Free Edenia is just an euphemism. Rain explains it very well.
He and Tanya ally with Mileena just to exploit her and to kill Kotal.

"Evil" is a stretch but she certainly isn't "good". Her tactics are ruthless and she's a user. It's strongly implied that she's sleeping with Mileena in order to manipulate her and secure some favor. Mileena thinks she has herself a fiery little concubine while in actuality she has another spy in her midst.

Tanya's ending even shows that she's using Rain to her own ends, including sexual manipulation. Rain is, if I remember correctly, descended from royalty (not necessarily Kitana's family) and is a crown prince. Their goals are the same but they aren't aligned. Rain wants the throne of Edenia, but so does Tanya. Rain may have a right to it technically, but Tanya's motivation is entirely selfish. This will probably lead her to make worse choices and forge shadier alliances as time goes on.

Tanya's original motivation was similar but decidedly evil - she allowed Shinnok and his minions passage into Edenia, which threw the realm into chaos and that would have allowed her to usurp the throne. I'm most interested in learning WHY Tanya is the way she is. Was her family or specific ethnic group/city/etc targeted by the royal family? Why is she a traitor, and if she always had those motivations, what ultimately precipitated them?

Hopefully they'll expand upon her in the next game.