These contradictions give rise to and were eventually resolved through class struggle. One of the many contradictions of capitalism was that capitalism tended to concentrate capital in fewer and fewer hands yet at the same time workers were concentrated together in ever greater numbers. So the glaring private nature of ownership of the means of production was contrasted with the social nature of production. The working class, herded together in huge factories and cities, grew in strength. This, amongst many other contradictions, would eventually lead to revolution.
History is for many simply the actions of great and charismatic men and women.
Marx never denied that such factors played a part in history. He fully realized that accidents and even charismatic people could help to either speed up the process of historical change or even slow it down. Certain things may well occur that throw a society back in the course of historical development. For example, the proletariat may find that its leaders are incompetent or the opposite may occur as in the case of Russia where they were very professional. Yet for Marx history is linear, it is heading towards an end goal.