Conrad Ellam would not describe the bloody scene at the south London flat where he found Samantha Bisset and her four-year-old daughter, Jazmine, on Thursday.
Appealing at Plumstead police station for help to track down the killer, he said: 'They had been brutally murdered by a psychopath and I am still in a state of shock from what I saw.'
Ms Bisset's mother, who had to identify her body, suffered more, Mr Ellam, a factory worker, said. 'I don't think she'll ever recover. I think it was a madman, a psychopath, who was possibly local, possibly had been watching the flat.'
Detectives believe Ms Bisset, 27, was murdered on Wednesday night. Neighbours at Heathfield Terrace, Plumstead, report hearing a man and woman arguing. Mr Ellam discovered the two bodies the next day.
Ms Bisset had been stabbed more than 70 times with a seven-inch blade and dismembered, detectives said. They fear the killer may strike again. Det Chief Supt Bill Ilsley, leading the investigation, said: 'The injuries were so horrific it would be very difficult to identify whether Samantha was sexually attacked or not.'
He also believed the killer could have been someone Ms Bisset knew, and that Jazmine would have been able to identify. Mr Ellam has been eliminated from the inquiry as has Jazmine's father, a New Age traveller living in south-west England.
Ms Bisset had complained of a prowler staring into the flat in the week before the killing. A couple walking a dog near by also saw a man watching the flat.
Mr Ilsley said: 'The person who has committed this terrible murder must have been covered in blood. There must be someone who knows who he is.'
Ms Bisset was described as someone who had travelled extensively before she had Jazmine. She then devoted her life to her daughter.
Mr Ellam said he had known Ms Bisset for about two years and was almost a father to Jazmine. He appealed to the killer to write to him, care of the police. 'Tell me why you did it because I think you're a very sad person,' he said.