A man notes that a certain ex-model/playgirl is worth a half a billion dollars. She's also promiscuous so bedding her is no problem. He devises a plan to get the money.
All he has to do is get her pregnant and wait for the baby to be born. When that happens he poisons first her son (legal heir) and then her. Maybe even the poisoning of the son was an accident -- a poisoned chocolate he expected her to eat but that her son ate instead, for example. (There are several poisons that are not able to be detected after a very short time in the human body.)
He whisks the baby away and all he needs now is a DNA test that proves he's the father. Voila. . .1/2 billion dollars.
If anyone's interested in poisons and their effects. Writer's Digest has a book for writers that covers the subject pretty thoroughly. Even ground up potato sprouts would do it.