My latest attempts at killing sims was in San Myshuno where he couldn't make a pool to drown them in (not yet rich enough for penthouses). Tried making them eat pufferfish of low quality and they survived that three times in a row. Then had them play with fireworks in a small room and that did the trick, except my main sim walks into the room and sets himself on fire too. After a bit of swearing I made him extinguish himself and get back out of the room, target sim soon burned to death though with flames not nearly as big as my main sim had earlier.