Kotzé is accused of hiring three men to gang-rape and mutilate his estranged wife, and of killing her 19-year-old son.
Tutu wrote a letter to The Star, in which he condemned what had happened, but also expressed his concern that the widespread outrage over Kotzé's alleged actions had led to him being dubbed a monster.
'Capacity to become a saint'
"He may indeed be guilty of inhuman, ghastly and monstrous deeds, but he is not a monster. We are actually letting him off lightly by calling him a monster because monsters have no moral sense of right and wrong - and therefore cannot be held morally culpable, cannot be regarded as morally blameworthy," Tutu wrote.
He added that Kotzé "remains a child of God with the capacity to become a saint. This may shock some of us".
"We should condemn ghastly acts of awful cruelty but we must, as they say, hate the sin and love the sinner, or hope that he may change for the better."
He said this was in many ways the basis of the Truth and Reconciliation process.
"We heard bloodcurdling stories of how people had been murdered brutally and yet we saw some extraordinary scenes of magnanimity when perpetrator and victim or relative of victim embraced publicly.
"If it were true that once a murderer always a murderer, then we should have had to shut up shop straightaway. But we believed then, and I hope we still do, that it was possible for people to change for the better, that the worst criminal could become a good and virtuous person."
Tutu was not defending Kotzé's alleged acts, saying decent people are rightly appalled at what had happened, and that it was quite right to condemn the "dastardly, barbaric and monstrous" deeds.
He expressed his deep sympathy and condolences to Kotzé's estranged wife and pleaded that people stop calling him a monster.
However, The Star quoted criminologist Dr Jackie de Wet, from the University of Fort Hare, as saying that the chances of someone like Kotzé being rehabilitated and becoming a good and virtuous person - if he did indeed commit the crimes he is charged with - are "very, very slim, almost minuscule".
ANC Women's League protesters outside the Modimolle Magistrate's Court during his appearance last Friday dubbed him a monster. "No human being can do such a thing," said ANCWL Waterberg treasurer Joy Matshoge.