Oh, Savathun was evil alright. The question is that she's a moving target, and we're not entirely sure where she's going to land. She realized that she made a bad deal with the Worm Gods, and wanted out of it. Her pride made it that she refused to serve the Witness, and apparently played a role in the allowing the Traveler ---and us---to survive the Collapse. The conflict between Light and Dark isn't so much one of good vs evil....but more of creation vs destruction. Order versus Chaos. Harmony vs Conflict. Life vs death. Its more about trying to find a balance of Universal Forces. Savathun is in the awkward position in terms of being evil (but unwilling to serve the even-more-evil Witness)....but also having a love of life. That was rekindled by getting to experience life without having to constantly kill while she was occupying Osiris' body. She was also willing to give up everything...and die...to be free of the Darkness and her deal with the Worm Gods. Which is why I believe that the Traveler rewarded her with being turned into a Lightbearer. She understood that Savathun had been tricked into aligning with the Worm Gods....and could be a valuable ally in the coming conflict with the Witness. The question is whether she and the Lucent Hive will be willing to join the allianace.