Cassandra, a trojan princess, who could predict the future but was cursed so that no one would believe her. Which leads to troy's fall.
Clytemnestra was the wife of Agamemnon. While he was away at war, she took Aegisthus as her lover. When Agamemnon returned from Troy, she killed him to avenge the sacrifice of their daughter Iphigenia. Later, her son Orestes killed her in revenge.
I also have a favorite romantic myths like:
Eros and psyche
Orpheus and Eurydice
Apollo and Hyacinthus
Achilles and Patroclus
Penelope and Odysseus