How do you kill Grim Reaper?

Answer 1

You don't. But sometimes if you have another sim around, it can save the sim that the grim reaper is after. (If they like eachother) To avoid the grim reaper, just keep your sim happy and healthy.

Answer 2

you can't unless you have a mod. the only thing close to a grim reaper would be the wither boss which is only available it the 2 or 3 lastest snapshots which you can download on the minecraft website or through wikipedia.