Final answer:
Medea's cruelty is shown in her betrayal and murder within her own family, and by sending a poisoned garment causing fatal consequences for Jason's bride and King Creon.
Step-by-step explanation:
Before the events of Euripides' play Medea, Medea demonstrates her cruelty through two ghastly actions. First, she commits a treacherous act against her own family by aiding Jason and then turns against her brother, plotting a dreadful banquet that reflects premeditated malice. Second, she manifests her vengeful nature by sending a poisoned cloak to Jason's new bride, resulting in the gruesome deaths of both the bride and her father, King Creon, thereby exemplifying her capability for calculating and severe cruelty.