CraigsList killer kept mementos of sex slays!

While his fiancée was going forward with their wedding Phillip Markoff, accused Craiglist killer was putting the finishing touches on his own collectibles – the panties of his victims, cops revealed. 

PLUS: his oblivious fiance Megan McAllister who had been out of the Boston area had received a warning from he own mother that a sex killer was loose in Boston.

"My mother warned me about that," Megan told cops after she and her boyfriend Markoff were brought in for interrogation.  Little did she know it was the man she was planning to wed.

Markoff was charged and held without bail in the Boston hotel murder of hooker Julissa Brisman, as well as robbery assault on escort Trisha Leffler.

A dedicated gambler, Markoff, may have turned to robbery to offset mounting losses, cops believe.  

And add to his growing collection.