Final answer:
Thomas Morton upset the Pilgrims by establishing the Maypole at Merrymount, trading guns with the Indians, and having liberal social practices, contrary to Puritan norms.
Step-by-step explanation:
The student's question refers to Thomas Morton, a colorful figure in early American history who often clashed with the Pilgrims. Morton was well-known for not adhering to the strict religious and cultural norms of the Pilgrims.
His most notorious act that upset the Pilgrims was the establishment of the Maypole at Merrymount, an act they saw as idolatrous and contrary to their Puritan beliefs. Morton's behavior, including trading with the Indians for guns, living communally, and his liberal stance on social issues, angered the Plymouth settlers who were seeking to keep a tight, religiously orthodox community.