You Can't
Chat with our AI personalities
No because in the beginning you choose if your sim wants to be either fat, skinny, or medium (fat and skinny)
no, there is no murdering in the Sims
Yes they can get skinny! Buy an exercise machine or go to the gym. Your Sims will wanna become skinny. It will increase their life happiness! Wanna help out? Add more!
You can not make a Sim anorexic.
My guess is that you can't make your sim to a vampire on the sims 3.