Matricide is the act of killing one's mother. As for any type of killing, motive can vary a great deal.
Known or suspected matricides
Cleopatra III of Egypt was assassinated in 101 BC by order of her son, Ptolemy X, for her conspiring.
Ptolemy XI of Egypt had his wife, Berenice III, murdered shortly after their wedding in 80 BC. She was also his stepmother, or perhaps his mother.
In AD 59, the Roman Emperor Nero is said to have ordered the murder of his mother Agrippina the Younger, supposedly because she was conspiring against him.
Mary Ann Lamb, the mentally ill sister of essayist Charles Lamb, killed their invalid mother during an episode of mania in 1796.
Sidney Fox, a British man killed his mother in 1929 to gain from her insurance policy.  He was convicted and hanged the following year.
The Parker-Hulme murder case of 1954. This case was chronicled in the film Heavenly Creatures.
Jack Gilbert Graham killed his mother along with 44 people by p