Marx.
His utopian world view brought more damage to the world than any other ideology except for national socialism.
Marx saw poverty and despair and wanted to change it for good, but good intentions rarely bring about anything good, as a saying goes.
One of the big problems with Marxism is its theory of economics. Giving the means of production soley into the hands of society, which translates to government, doesn't encourage growth and innovation. The self-interest of the capitalist to make money provides the population with useful stuff such as smartphones, TVs, etc. We wouldn't have those things if not for capitalism.
Also through constant competition it is ensured that corporations invest in research to be one step ahead of their competitors which leads to a fast technological advance.
The wealth of the western world that came along in the 20th century was not achieved through labor unions or taxing the rich, but by price-reduction of industrial goods.
For example, the average income per household didn't increase since the 80s in Germany, although the spending capacity is now much higher than back then. That can only be traced back to the cheaper production of goods.
In marxist planned economies such as Soviet Russia or Cuba all those things didn't happen, or weren't allowed to happen if you will. They slithered down, after an initial increase of wealth through redistribution, into even more poverty because the slowly growing economy couldn't supply the growing population.
So please, any hipster commie who takes pride in how "smart" he or she is because he has read the communist manifesto (I know a few of these people) or the girl who tweeted a photo of her holding a sign "resist capitalism" shot with a brand-new iPhone, grow up.
If you lose your job nowadays you get on welfare. Poverty has literally been been eradicated in the western world, there's no need for a proletarian revolution.