She learned what Salem tried to teach her in Vol 4 & 5. That your enemies (Watts and Neo) can be as useful as tools as your followers.

As for Watts. He would have blabbed about her using the Jinn's last question if he had lived or at least held the knowledge about how casually Cinder used it to get leverage over her.

Killing him means that she is free to tell Salem that Ruby used the last question or any other story that she needed to improvise to match the situation.

If her plan had gone completely flawlessly she would have killed all the witnesses to her misuse of the lamp, killed those meddling kids, acquired two relics for Salem to earn her way out of the doghouse and bagged the winter maiden powers for herself to boot.

As for Neo it was always an alliance of convenience and she was probably planning to kill her at some point anyway, as Neo no doubt planned to do to her if she had the chance.