Socialism views human nature as co-operative, positive and altruistic. They believe that humans are born equal and that differences in their success are created by society and nurture, not nature. As such they believe that equality within human society is both normal and should be encouraged as humans are collaborative and that co-co-operation will create the most progress for everybody than individualistic competition advocated by Liberal theories. So this reinforces the argument that for some socialists that there should be equality of outcome for humans as well as equality of opportunity. However within the present system of economics and society there is vast inequality. As such Socialism has been linked with the removal of capitalism as it's the present system of economics.
Capitalism is a system of economics which relies upon consumption and the creation of wealth. It believes that the market based upon demand and supply will provide everybody with what they need in the most efficient manner, and that competition will spur individuals to create wealth for their own individual use and that overall the combined effect of everybody aiming to create wealth for themselves will result in overall greater prosperity for everybody. Socialist disagree with capitalism on numerous fronts. It places a huge emphasis on private ownership and property which socialists strongly disagree with as private property encourages engrained inequality due to hereditary nature of property ownership. Socialist would argue that their should be collective ownership of the means of production as this would result everybody benefiting from the production and abuse and license would be removed. They also disagree with the capitalist belief that individuals competing will produce the best results, they argue that this isn't efficient and creates waste and that it encourages a distortions in the humans natural nature which would be to co-operate and aid each other to create the best possible outcome.
The key reason why socialists feel aggrieved with capitalism is it creates winners and loser out of a society where they believe there isn't such a distinction between each individual person, that justifies such inequality. Marx especially was damming of the capitalist system. He took a highly different analysis to how the system operates than how liberal theorists examine capitalism, he said that the history of capitalism has been a continual class struggle with the people who have power and wealth oppressing the majority and forcing their ideology onto the majority, This is achieved because the ruling class own the means of production and the workers who actually produce the finished product don't as such the ruling class can extract the "surplus value" that workers create by reforming inputs into more valuable outputs "Capitalist production, therefore, develops technology, and the combining together of various processes into a social whole, only by sapping the original sources of all wealth - the soil and the laborer". As such this creates in the inequality which is unfair in human society, but this hasn't been overthrown by the majority because the capitalist system creates a "false consciousness" which means that any individual can see their life's slowly improving and can find people who are worse off than they are and as such they believe that their position within society is deserved and just, when in fact its the abuse of capitalism that means they don't have just rewards. Marx says that due to this inbuilt inequality and due to the need to consume and produce to create wealth the market will always over compensate which will result in boom and bust cycles which will always fall worst on the working class as they don't own the means of production, he believes that the only possible solution is to overthrow capitalism in revolution. Then form a new society on the