Despite the lowly position he occupies in what was then Southern Rhodesia—modern-day Zimbabwe—Moses is a very strong character with a mind of his own. To a considerable extent, his character has been forged by the education he received from white missionaries. Because of this, he is able to gain a broader perspective on society and how it treats the Black majority.
Inevitably, there is a limit as to how much space and agency Lessing can give the character of Moses. In order to give a realistic account of life in Southern Rhodesia, she has to show the many legal and social restrictions to which Moses, as a Black African, is subject.
At the same time, however, in her portrayal of Moses, she gives us a picture of a man whose intelligence, fortitude, and mental strength enable him to defy some of the conventions of a society that keeps him in a state of subjection.
He understands all too well the rampant injustice and prejudice that the ruling white minority inflicts upon the majority-Black population. It is this realization, more than any other, that leads him to commit the ultimate act of resistance: stabbing Mary to death.
In the aftermath of this shocking crime, Moses calmly hands himself in to the authorities, an indication that, in killing Mary, he knew exactly what he was doing and was freely exercising his agency as a human being.