I believe Emeric Pressburger originally had a slasher in mind, that's to say a clothing slasher but it was considered too morbid (shades of Peeping Tom Powell's later film). It may have been based on hearing about this incident: http://en.wikipedia.org/wiki/Halifax_Slasher
You can imagine the pitch -
"We need a propaganda film to keep American pubic opinion behind us in this war, shared values, that kinda thing. Any ideas?"
"Yes. There's this man who lives with his mother, gentleman farmer type and he slashes girls and..."
"Did you say slashes. No, no that'll never do, there's a war on you know."
"Well okay he pours glue in their hair while firewatching."
"Why'd he do that? Sounds a rum cove. Fifth columnist?"
"Not at all. He wants to keep the GIs away from the landgirls so they can attend his lecture."
"Why shouldn't they learn some rhythm music from the yanks, hot work piling hay. Broad minded modern girls and so on. What's his lecture about?"
"The pilgrim's road."
"I see. He gets his comeuppance I assume?"
"He becomes a sort of saint, handing out blessings."
"Fine. Sign for the budget on the way out."
Them were the days.