This question refers to Orestes, the son of Agamemon and Clytemnestra. After Agamemnon returned home from the Trojan War, Clytemnestra (aided by her lover Aegisthus) killed Agamemnon. Later, Orestes took revenge and killed both Aegisthus and Clytemnestra. In Book I of Homer's Odyssey, Orestes is held up as a model of dutifulness for avenging his father's death. Later traditions, however, are not quite as kind to Orestes as the questioner suggests. Orestes appears as a character in Aeschylus' Libation Bearers and Eumenides, Euripides' Electra, Iphigenia at Tauris, and Orestes, as well as Sophocles' Electra. In each of these plays, the tragedians raise questions about the necessity and rightness of Orestes' killing of his mother. Eventually, though, Orestes does escape "demonization." We should also keep in mind that Orestes killed his mother in response to an oracle from Apollo, which commanded him to kill her.
In contrast, the killings committed by Medea did not have divine approval. Because Jason divorced Medea to marry King Creon's daughter, Medea killed not only the princess (and Creon who is an inadvertent victim), but also ends up killing her own children. Whereas killing one's mother is a horrific crime, a mother killing innocent children is usually regarded as a more heinous crime. Also, from the Greek perspective, the fact that Medea's killings were not sanctioned by the gods is also a strike against Medea.