I've just done it before by accident (okay, maybe three or four times), but the animations did give me a hoot. Their facial expressions were priceless. XD

You can kill a sim by building 4 walls around it without any windows or doors in them. I did this the other night. While she was dying she actually got a moodlet that she was in a bad mood since her space wasn't decorated nicely....go figure. It took her almost 3 sim days to die though.

In the first two Fallout games, children in fact could die. You could even kill them yourself. You would have half the game's NPCs out for your blood for being a "Child Killer", but you could do it.

That's only babies and toddlers. Kids can die.
I found that feature very annoying - when I was "cleansing" a Neighbourhood the other day, I had to wait for the social worker to come and pick up the kids before I'd get a nice empty house that was fully furnished.

Sims can drown if you block all the pool exits. I killed Lisa Bunch about a day after she gave my favorite sim a son, mainly because she was grating on my last nerves and it was an 8-sim family by that point. She's my first kill. I blocked her in with the buffet table and a coffee table that she swiped from the Goth's yard the day before. She had a moodlet for about an hour that said "drowning" (threw her in while she was exhausted) and then she finally kicked it. Grim took forever to make an appearence, and her husband saw him and promptly fainted. After she was finally dead (and everyone cried, and I cheered enthusiastically) her widowed husband, who didn't even seem aware he was a father until that point, ran upstairs and started spamming his son with snuggles and playing and got a several wishes to play with his son and teach him different things.

Also, in an incident I didn't save after, my sim (same one that was married to Lisa) decided to perform an experiment on his bed, and he ignited it. His room filled with flames, and while he ran into the downstairs bathroom and fainted, adorably, everyone else in the entire family insisted on cancelling out work and school, and instead filed into the room. Only one person in the room survived, because she was smart enough to not run right into the fire.

Ghosts that died in fires tend to take baths or showers at any opportunities. ANY OPPORTUNITY. Sometimes I have sims needing to shower but they can't because there's a ghost in the bath. And they never leave.

I thought they could drown even with the exits intact? I had my sim go swimming in a public pool and it said he was drowning because so many of his needs were really low...he must have been too tired to swim.

Well, I couldn't get my sims to STAY in the pools: the second they jumped in while exhausted, they jumped right back out. In a larger pool, after swimming a while, they might drown themselves, but none of my earlier attempts managed to work.

it works if you draw walls around it, i think. Or if they are hydophobes.

Do you have to be in the same household with the dying sim in order to make a ghost of them? I witnessed two deaths of relatives already, but the sims left no pile or grave after them, and nobody called me from Science Lab with offering to make them ghosts.

The pool trick works, I was playing with the Landgraab family and Malcolm was in he pool, I got busy with the adults and then the Grim Reaper appeared with that spooky music! :'( I had to reload my game x'O

Have you checked the mausoleum? If they didn't live with your current family, but you family knew them the game places them there. Just go to the graveyard and click the mausoleum and "manage dead" you can drag the tombstone out and place it in the graveyard or into your sims inventory and bring it home.