what does the creature say made him a 'fiend'? what is romantic about this?

The monster eloquently argues that he is intrinsically good, full of love and humanity; only the greatness of his suffering has driven him to commit acts of evil. Though he is surrounded by examples of human happiness, he finds himself excluded, through no fault of his own, irrevocably excluded from such bliss. He implores Frankenstein to listen to his story; only then should he decide whether or not to relieve the creature of his agony.

Shelley suggests that the creature's misdeeds are caused by the enormity of his suffering; at heart, he is essentially good and, more importantly, essentially human. If he is monstrous, no one but Frankenstein is to blame. When the outraged creature demands of his creator, "How dare you sport thus with life?," the question succinctly represents the sentiments of the reader and perhaps even of the author as well. Frankenstein, in his hypocrisy, longs to murder a being who owes its life to him. If the creature is, paradoxically, both inherently good and capable of evil, then his creator is as well.

THe creature's isolation and emotional distress made him a fiend. He is unloved and spurned by the human race so he will exact vengeance on them.