Mr. and Mrs. Cutter are just about the most unpleasant couple you will ever meet. Mr. Cutter cheats on his wife constantly and connives to make sure that her family won't get any of his inheritance after he dies. Mrs. Cutter delights in arguing with her husband and making him unhappy.
If nothing else, the physical description of these two should make it quite clear that they are just plain unpleasant". Mr. Cutter is has a "pink, bald head" and "yellow whiskers, always soft and glistening. It was said he brushed them every night, as a woman does her hair. His white teeth looked factory-made. His skin was red and rough, as if from perpetual sunburn; he often went away to hot springs to take mud baths" (3.11.3). If that sounds unappealing, check out how Cather describes Mrs. Cutter:
[A] terrifying-looking person; almost a giantess in height, raw-boned, with iron-grey hair, a face always flushed, and prominent, hysterical eyes. When she meant to be entertaining and agreeable, she nodded her head incessantly and snapped her eyes at one. Her teeth were long and curved, like a horse's; people said babies always cried if she smiled at them. Her face had a kind of fascination for me: it was the very colour and shape of anger. There was a gleam of something akin to insanity in her full, intense eyes. (2.11.5)
Mr. Cutter fills the villain role in My Ántonia rather well; he tries to have sex with Ántonia and he beats up Jim. He's also a money-lender, which means Cather pegs the difficulties of the poor on his greed. Of course, in this novel, where the heroes are rewarded and the villains punished, the Cutters meet a grisly end in the form of a murder-suicide.
The Cutters are also based on characters from Cather's past, named Matthew and Agnes Bentley. Matthew was, like Cutter, a greedy moneylender, and he also impregnated several hired girls, as does his fictional character. The crazy story of the murder and suicide is also, amazingly, based on history (source: My Ántonia explanatory notes, Oxford World Classics Edition, 2006).