Capítulo 801: Beautiful Network.

In killing Cirk's mother, the sword stab by Melinda had been mostly a medium to allow her to drain the woman of all of her life essence thereby making her 'dead'. 

Once the purpose of the killing (to completely break Cirk mentally) was complete, there was no reason to let her remain dead and Melinda promptly returned her life force which essentially brought her back to life.

Cirk's mother might have made some comments and was clearly very opinionated about Melinda's exile while supporting whatever her darling son did, but Melinda saw no reason to kill her. She still had restraint.

Plus, letting the mother live on to forever mourn her son seemed a good enough punishment. This was supported by the fact that Cirk's mother saw being alive and seeing her son's dead and tortured body to be immensely painful and promptly broke down the moment she could observe her surroundings.