Mairwyn was a time traveller.
Calling herself "Beatrice", she stole a vortex manipulator from Captain John Hart. She used it to travel back to 2009 Cardiff. She tried to manipulate Ianto Jones into travelling to an even earlier time and stealing technology from Torchwood One, with which she could take over the Earth. Knowing events in Ianto's future and his soon-to-come death, Mairwyn thought he would be the easiest to exploit.
She used a device to show Ianto his future, and the world that would be created if he accepted her offer. He decided not to do what she wanted. He killed Mairwyn to prevent her trying to use someone else for her purposes, then retconned himself.