What can I do about my younger sister?

Lately, my younger sister (21) has just been so mean and unforgiving to everyone. She says that she can't stand people that aren't as smart as her or that people give in to their emotions "like the disgusting pigs they are". She believes that their is no such thing as being left-minded or right-minded and that the people that say they are just suck at stuff and need to become smarter. She believes that only "retards" let their emotions affect them and that you need to approach everything with cold hard logic. She says that the people that can't apply logic to this world deserve to suffer and that only the smart deserve to succeed.

She says that only like 10% of the world deserves to lives and that everyone else is a waste of space and should just die. She says that she can't stand how people let themselves have jobs that shouldn't even exist in society but do because people are such idiots. For example she thinks that there shouldn't be any fast food or any workers for fast food because if people realized how stupid you have to be to work at one of those place or that how bad the food is for you then they wouldn't create such a thing in the first place.

She believes that we need to do away with all our technology and should go back to basic agriculture and hunting/gathering. She said this way only the physically fit and smart people that are able to adapt will be able to survive. She believes that anyone with physically/mental disabilities should be gotten rid of because they pollute the world by making people take care of them when they deserve to die since they can't support themselves.

She keeps saying that she wants to either move away to an isolated part of the world to get away from everything or she wants a plague/war to come and kill almost everyone so that modern society is in shambles and it goes back to the hunter/gatherer lifestyle.

She has been this way for about the past 6 months and we don't know why. She went to a psychiatric hospital a few months back but left after three days since she checked herself in voluntarily. She said that everyone there were morons because they let themselves believe they had a mental illness. She said she went there because she wanted to see how society reacted to her views and how they would try to "get her to change them in order to adapt to society". She said after they told her they wanted her to listen to people "who were undeserving of even the most basic respect because they weren't intelligent" she realized that they wanted her to become like everyone else which is something she wouldn't ever do.

Also she has been driving very very aggressively ever since she started getting these viewpoints. She said "So what if I drive fast and kill someone? They knew the risks when they got behind the wheel. If they end up dying then that just shows that they weren't meant to survive in this world anyway." This is a very dangerous way of thinking and we are all worried for her.

What can be done about this? You can't check someone in to a mental facility unless they go willingly and obviously there is no way she would go to one. We are worried for her and want to help her but don't know how.