One of the components of Marxism that led to much of its later confusion can be seen in Marx’s rejection of ethical idealism, a strain of Marx’s thinking that is too often disguised and unaddressed, and which invariably takes its idealistic fans by surprise, if it enters consciousness at all.
The grounds for all of this, beginning with Hegel, are complex, but finally stem from an inability to really grapple with the transcendental idealism of Kant.
From Sidney Hook’s From Hegel To Marx, Columbia, 1994, p.47
Rejection of Abstract Ethical Idealism
Opposition to the excesses of rabid individualism, however, comes not only from social realists but even more often from ethical idealists. Indeed it is in this latter form that it is most vocal and emphatic. What age has not heard a cry for justice in the name of Christ, Kant or some other ethical prophet? When has not someone’s conscience, someone’s devotion to things invisible led to conflict with the social order? And yet both Hegel and Marx regarded abstract ethical idealism, whether it have its roots in Christ, Rousseau or Kant, as Utopian and unreasonable-as even more dangerous than any philosophy of social atomism. It involved the same denial of the priority of the group over the individual; the same acceptance of the natural rights, or conscience, ethics which lies at the basis of philo¬sophical anarchism. Because it endangered the stability of the state, Hegel stamped it as a revolutionary doctrine; because it hampered militant class activity, Marx condemned it as Incipiently counter-revolutionary.