I can understand witnessing a death (even of a Sim they don't know) could make them upset for a few hours. Days on end is frustrating.
I like to play with aging on normal or fast. (Most of my families tend to be ghosts, vampires, or both, so the aging is really just to get through toddlerhood and childhood.) But since my Sims aren't aging like everyone else, they are constantly getting phones calls about the random NPC who died that they only met once. I think they're sad more days than not. And it doesn't seem like there's any workaround. I've tried using vampire powers to manipulate emotions, potions, and the sadness alleviation lotion. When they're sad about death, they don't like to let go of it.
Only vampires with dampened emotions seem truly unaffected by it. And they're not affected by, well... anything.
Seems the best solution is just insist they only socialize with other immortals. >.<