Forced to drown baby.
But Algarron, 26, became more and more jealous at the devoted attention his lover was giving to little Catherine, two.
Over a number of months, as they lived in Paris, Algarron - who was obsessed with the perverted works of the Marquis de Sade - coerced Labbe, 30, into showing her love for him by killing the child.
After a number of botched attempts, including throwing the child into a canal, only for her to be saved by a passerby, the simple-minded Denise was eventually persuaded to plunge the tot head first into a filled zinc washtub and hold her there until she was dead.
Full of admiration, Algarron would later tell a friend that "it takes courage to kill your own daughter".
Algarron: 20 years