Aren't those normally unkillable?
some "essential" characters can die, but any main quest's character(s) can't die until they're no longer essential
for example, that girl you talk to in the Hall of the Dead during Namira's daedric quest can die
really easily
it's quite an annoyance
I had to start a new game because a dragon attacked riverwood and killed a few people plus a key NPC.
a good thing to remember is that if someone
can die, you don't need to start a new game just because they die