Yeah, Murphy's Law alright.
My brother had an evil vampiress who was trying to commit murder, so she could see another sim die. She tried everything--draining the victim's blood, draining her hunger meter, humiliating and angering her, using the voodoo doll on her, but nothing worked. (It was actually pretty funny. Sometimes the voodoo would backfire on her! But she kept trying...) This was an elderly sim, but she survived everything an evil master vampire could throw at her.
Finally he gave up, and made a level 1 vampire just so she could murder him by luring him out into the sun. (That wasn't easy either, but she was able to kill him eventually.)
So murder is very difficult in this game. BUT, I've had several of my favorite sims die on me.
One went swimming while she was tired, and apparently fell asleep in the water and drowned. I didn't even know she was swimming, until I saw a message that she was dying. We tried to save her, but the Reaper just came and swept her up without giving her family a chance to beg for her to be spared. (I later played her as a ghost, but she leaves puddles of water all over the house!)
My favorite, and most powerful, sim was killed by a cowplant. You would think a powerful sorceress would know better than to eat the cake! Why did she do that? I didn't make her do that. She just did. I was shocked. Her family didn't have any death flowers, and she was the only caster in the household that could revive the dead. Fortunately the Reaper spared her life when her little girl begged him to let her have her mommy back. (Or MAYBE the real reason was because he had strong green and pink bars for the mom before she died, since she was always flirting with him? He's never heard the pleas of any of our other sims pleading for him to spare people.)
Later I had another sim die in a pool, while she was doing a backfloat while tired. I wasn't sending her into the pool, but she kept going back in. I was making her get out of the water and sleep in a bed. But she kept going back into the pool, until she drowned. Her friend's husband was able to save her with a death flower though. (Then I roleplayed her falling in love with him, even though I was roleplaying her as a lesbian before that. I figure the near death experience could inspire certain feelings.)
Then I had another sim die of hunger. I didn't even know she was hungry. I was playing a different sim that was visiting her house. Then I saw this sim wake up, get out of bed, walk about 2-3 steps, and die. I didn't even know why she died! Grim Reaper came. The character I was playing begged for her to be spared, but he took her anyway. (I don't remember if I had a death flower or not.) I logged into her game to play her haunting the house. Her lover asked her to join the household, and she immediately said yes. Her traits showed that she had died of hunger.
And just a couple days ago I had a high level sorcerer, die from laughing himself to death. I think I'll play him as a ghost too. I just need to decide if he should continue haunting the house he died, or have a family member move his urn to one of their own lots. I think I'm also gonna have my eligible bachelors pursuing his wife, now that she's a widow.
So murder is hard, but your favorite sims seem to die pretty easily doing stupid things on their own. Murphy's Law.