Let me godwin that for you: Hitler was elected by proportional voting. Nazis rose to power in Germany in one of those convoluted negotiations that countries with proportional voting do all the time.
An extremist has little chance of being elected when candidates are chosen to represent a district. In any given geographical area, there are different sorts of people, therefore moderates are much more likely to get elected.
Under a proportional system, things are different. It's easy to find enough supporters for any extremist view, if you count votes all over the country. And when you try to make a coalition with extremists, who do you think will end with all the power?
A coalition government often has 45% of moderate politicians for one side, 45% of moderates for the other side, and the power ends in the hand of the 10% of extremists who can choose to support one or the other side.