You know, I initially thought Selene's goals were a bit confusing, but they actually make sense
Selene loves distortion, which is another way of saying she loves chaos. She basically loves being entertained by insane shit happening and believes that chaos leads to more powerful individuals. As well, Selene believes the era of Dragons is over and has agreed with Acnologia that having all of the Dragons disappear is what's the natural next stage of the world. She believes that Humans are the future and being able to rule over Humans is what will give her the ideal world. So Selene basically wants to live in a world with no Dragons in it and rule over all of the Humans as the strongest being in the world. The issue with this is that in Earthland, there are 4 other Dragon Gods equal to her, and she alone is not capable of taking out the other Dragon Gods, which made her plan to take out the other Dragon Gods via outside means. So she traveled to other worlds in hopes of finding people who could erase magic power so that she could have the Dragon Gods magic power get erased and be defeated. Eventually she found Elentir and Faris, and learned that she could erase magic power, so she sent her on a mission to take care of the Five Dragon Gods in Earthland. But eventually, Selene came to like living in Elentir and realized it was the world that she was looking for. Elentir is a world devoid of dragons and only has Humans primarily, so she wanted to rule there. At this point, she basically gave up on defeating the Dragon Gods.
Faris was still on her mission tho, but Selene had given up on this mission at this point, so just for fun, she had Faris awaken Aldoron and said that he can have Earthland, cause now she has Elentir. When Aldoron was defeated, she finally said she will take over Elentir and obtain the ideal world devoid of Dragons that she wanted. However, after fighting Suzaku and coming to see how powerful Humans have become over the years, she realized now that Humans are much stronger than before, they will finally be the key towards tipping the balance of the Five Dragon Gods in her favor, which basically reinvigorated her goal of killing the other Dragon Gods and making Earthland her ideal world.
So she’s back on the mission to kill the other Dragon Gods now that she’s seen the powers of Humans and believes they will allow her to defeat the other Dragon Gods.