Deadly Grapefruit

Inspired by a murder in S1E3 The Great Game.
Michael Rudkin has a cruel son. One day he finally has enough of Alex's horrible actions and threatens to remove him from his will, donating the money to charity instead. Alex, a wealthy but deeply in debt pharmacist. is desperate for this money.
Determined to get his inheritance before Michael can rewrite the will, he kills his father with a glass of grapefruit juice.