Lydia only dies by your hand. When she kneels down, she is very susceptible to splash damage. So anything you hit close to her with a spell, especially fireballs, may hurt her. When she goes down, I run away and try to draw them away so she won't take any damage. Even with careful baby-sitting, she gets killed.

it's kind of your own fault for provoking them
they usually just go about their business if left alone
I usually kill them as well
just saying it wasn't exactly first degree murder here XD