Jack Campin has already mentioned my favourite - the wonderful 'Laird O' Wariston'.
Prince Robert's mother poisoned him.
Young Willie's mother didn't actually kill him, but she caused him to drown in Clyde's Water.
Then there are the two who took part in the Burke and Hare murders: (guilty before the fact)
Two women there were in the plot; the wife of William Hare,
The other named McDougal, and travellers they did snare,
They got them to the lodging-house, and when they'd drunken deep,
Hare and me, we smothered them as they lay fast asleep.
Jim Carroll