Speaking at the Edinburgh TV Festival, Ashdown said that Barbara Windsor's decision to quit the serial last year meant that there would have been no aftermath.
He explained: "We had a list of the cast and went down it to create that 'shiver' moment. Janine could have done it but she's killed before.
"But we settled on Peggy, but then we found out that Barbara Windsor had decided to leave. Like most of these ideas it just appeared as a notion... we settled on Stacey. It gave us loads of story afterwards."
Stacey Slater was unveiled as the killer. Lacey Turner, who plays her, will leave the programme in 2011.