The painting portrays a man bathing in the bathtub, and standing next to the man is someone holding a knife, getting ready to stab the person. I think it’s supposed to take place during the French Revolution. Can’t remember the title. 
Never mind. Found it by a simple Google search.
Just search “French Revolution stabbing bathing painting”.
The Death of Marat although you are probably thinking of this painting, called Charlotte Corday.