How do you kill off a non-playable Sim in The Sims 3?

Answer 1

Go inside their home, and make mac and cheese. While it's on the stove, stop cooking. After a minute or so, it should catch on fire. Then, unless they have a fire alarm, or something to put it out, they should die.

Answer 2

Step 1 - Get the offensive Sims to be a Good Friend of yours or better and have a really friendly conversation with them that goes well. Use the Friendly socials successfully multiple times to achieve this. If multiple Sims you want removed, you can invite them all over at once or throw a party (kicking out any party crashers) and use "Get Lucky" if on the Console version of the game, to get them to instantly become your Best Friend and gain a Friendly mood with you.

Step 2 - Ask them to Move In with you. To get this to come up, you'll have to be talking to them for quite a bit until the point they're really friendly and comfortable with you. Of course, you'll have to have four or less Sims living in your household for this to work. The less you have in your home, the more you can get to move in with you at once. Just know, if it is during a party, you can't get them to move in with you at that time, but you can get them to move in later. Just wait for the party to be over and invite them back the next day and talk with them until they're comfortable with you.

Step 3 - Once they've officially moved into your household, ask a genuine Good Friend (or better) to move in with you. As long as you have a Friendly mood with them as well, the Move In screen should come up again with them. Now simply move the offending Sims into the new friend's house and they will be gone forever! They never actually "move" into the other person's house; they just disappear from the game save permanently. Also make sure your actual friend does not move in with you, as they will automatically be in your family when the screen pops up. Just move them back into their own place or remove them from your family. Since they never actually moved in with you yet, they won't disappear unlike the offensive Sims who are already living in your household now. And you can also do this with up to six Sims in your household, as long as the final result isn't more than six Sims per household.

Repeat Steps One through Three for all offensive Sims you want removed from your saved game. Just keep in mind, once you remove an entire family from another neighborhood house, the game will just replace them with more randomly-made Sims who aren't supposed to be there, like the carpool drivers and the service workers, most of which have off-putting personalities and many of which are Insane or Neurotic (like all of the Pizza Delivery people). But you can do these steps as many times as you want to remove unwanted Sims from your game!