I would say Ja Kyung, she came in with ulterior motives against Hee Soo and she kinda seems like Ji Yong's ex/the mother of the kid?
so she's there to "take back" her place. or she kills He Soo. for now I don't see anyone wanting to kill Seo Hyun
the new maid, well I don't see anyone wanting to kill her yet either, Seo Hyun would hardly care, unless it's one of the other maids who gets jealous, like Hee Soo's assistant who's name I can't remember
in woman of dignity the writer had a subtle yet great murderer so I don't think it'd be anything too obvious or too twisty