*Rapist implies artistic skill. You don't have murderists.
...What? Maybe it's just me, but I don't exactly associate art with rapists.
According to wikipedia's dictionary, the ist suffix has two applicable meanings.
3) A member of a profession or one interested in something.
botanist, psychiatrist, one who studies psychiatry
4) A person who uses something.
violinist, one who plays a violin
The second meaning is less inappropriate, but (I'd argue) still wrong. People do not (usually) use rape- rape is the end, not the means. The first meaning is even worse. Rape is not a profession, something that you study and perfect. It's not like art or physics. Rape should have the simpler -er suffix attached to it, like with murder.
1) person or thing that (does the action indicated by verb); used to form an agent noun.
reader, cooker, computer, runner-up, do-gooder