Yes, the players may be sane and realize they should run away from a liche. But it is notorious that they often do not. Total Party Kill may be abbreviated to TPK because it's so commonplace in games, but it is a plot device of very precise and limited scope in a novel. Normally, you do it in the prologue and let the actual characters learn of it.
Players may even come up with a wild and wacky solution that you would never have thought of, and which you can't really use in a novel because it's too strange and incongruous, and doesn't fit the rest of the story