Where did Marx go wrong?

At the risk of simplifying: Marx erred in his view of human nature. Marx saw humans as having a nature that is social and productive. Various forms of impoverished economic and social life could corrupt this nature, with capitalism being the latest and the greatest of the corrupters. For instance, capitalism has the power to turn naturally productive humans into unproductive proletarians who are productive only through coercion (e.g.. through the threat of unemployment). Capitalism also has the power to turn naturally sociable humans anti-social, which is expressed in a variety of forms of conflict and violence.