Sjef Rijke seemed to have no luck at all with women. During January 1971, his 18-year-old fiancee, Willy Maas, experienced a week of racking stomach pains that climaxed in her death. 

The symptoms seemed to coincide with food poisoning, though friends and relatives could never be precise about the suspect dish.

At Willy's funeral, in Utrecht, Holland, Sjef was visibly distraught. His period of mourning was abbreviated by engagement to a female friend of several years, young Mientje Manders.

Near the end of March, Sjef's second fiancee complained of nagging stomach pains that quickly proved debilitating. Rijke sat beside her bed and held her hand, tears streaming down his face when Manders died on April 2. 

The authorities in Utrecht were concerned about the strange "coincidence," but Rijke seemed to have no motive for eliminating his fiancees. Flying in the face of grief, Sjef married only three weeks after Mientje's death.

It was another star-crossed union, marred by Rijke's pathological jealousy; he quarreled bitterly with 18-year-old Maria, prompting her to leave him, filing for divorce six weeks after the wedding. 

Curious detectives asked about her health, and they discovered that Maria had been subject to repeated stomach aches throughout her short-lived marriage; change of domicile had cured the problem overnight. 

A short time later, Sjef acquired a live-in lover who, in turn, began to suffer stomach problems. 

These were dutifully reported to her mother, who inquired about her diet. Sjef, the older woman learned, ate everything her daughter did, and seemed to feel no ill effects. It was discovered that his latest paramour was fond of eating peanut butter as a snack, between their normal meals, and samples from the jar were found to have a strange, metallic taste.

Delivered to a chemist for analysis, the peanut butter was discovered to be laced with rat poison. Still lacking any motive, homicide investigators searched for other suspects, grilling Rijke's cleaning lady. 

The investigation soon focused on Sjef, after a local merchant recalled selling him several boxes of poison in recent months. Under close questioning, Rijke broke down and confessed his crimes, describing the sadistic pleasure he obtained from watching women suffer. It was never his intention, he explained, to kill his fiancees; he simply loved to see them squirm. Found legally sane by court psychiatrists, Rijke was tried for the murder of Willy Maas in January 1972. 

Upon conviction, he was sentenced to a double term of life imprisonment.

Michael Newton - An Encyclopedia of Modern Serial Killers - Hunting Humans