Have you all watched until ep 12? If so I can tell you what happened until now *SPOILERS*
First, Min Woo didn't kill Eun Ha, the maid was putting some sort of drugs in Eun Ha's Wine which she AND Min Woo drank from so both collapsed, the sister came and Eun Ha fell in the pool and drowned to death, she didn't kill her directly but she didn't help her out. Since Min Woo also collapsed he couldn't remember anything. The sister cut the cctv out, Seok Hoon ordered the doctor to make Min Woo think ge killed Eun Ha (or choke her in this case) through hypnosis and he gave him a syringe before he hypnotisied him. We could see in the earlier episodes when Seok Hoon threatened Ji sook by taking her bag and choked her a bit to make a mark on her throat, Min Woo saw that mark and it made him believe even more that he hurt her. The medications that Min Woo had to take were discovered to consist hallucination drugs which made him "crazy", and there is a specific maid that is helping Seok Hoon by putting drugs in wines etc but in the latest episode we could se that the maid doesn't want to follow Seok Hoon anymore. Min Woo's sister realized that Ji sook is acting like Eun Ha when Seok Hoon visited "Ji Sook's" grave ( I think she heard when he said "Eun Ha") lastly Ji Sook knows what's going on so she is doing everything to stop Sek Hoon from planning to kill Min Woo because that was his and Eun Ha's wish, Ji sook finally discovered that out.
I realized that you all have guesses but what I said is true. PS it's the first time I'm keeping track on a dramas xP I mostly dont understand what it is all about butin Mask I feel like I understand everything xD