"From each according to his ability, to each according to his need" shows that Communism must lead to totalitarian regime. Atrocities of Stalin, Lenin, Mao and Red Khmers were not "errors", they were natural consequence of the essence of Communism.
Greed and laziness will ensure that most people would overestimate their needs and underestimate their abilities. This would require some external judgement, by the state, by the party, by the system. So, basically someone else would judge how much you can work and enforce that you work that much. Similarily someone would have to assess your needs and provide you with goods according to this. So the result is that an individual cannot decide for himself, which would be... slavery?
And in fact, apart from short periods of enthusiasm e.g. right after WW2, all communist states were founded on slavery, both hidden (compulsory employment) and official (e.g. Gulag).