The reasons why Charles I was guillotined are as follows:
1. Religious conflicts were particularly chaotic during the reign of Charles I. His subjects were generally distrustful of their king's faith, and his mistakes successfully aided the Protestant forces during the Thirty Years' War. Charles I also reused controversial church figures at the time. Many of his subjects believed that doing so brought the Church of England too close to the Roman Catholic Church.
Afterwards, Charles I also tried to force Scotland to carry out religious reform, which triggered the Bishops' War. All this strengthened the position of the English and Scottish Parliaments, and ultimately contributed to the demise of Charles I.
2. In the last years of Charles I, the English Civil War broke out between him and Parliament. At the same time, he formulated a series of religious policies, which aroused dissatisfaction among the Calvinists represented by the Puritans. After Charles I was defeated in the First English Civil War, Parliament wanted him to accept a constitutional monarchy.
However, Charles I persisted. He formed an alliance with Scotland and fled to the Isle of Wight. This behavior completely angered Parliament and led to the second English Civil War. Charles I was again defeated, and he was captured and executed for treason shortly afterwards.
Extended information:
Introduction to Charles I:
Charlie I (English: Charles I, November 19, 1600 - January 30, 1649) , also translated as Charles I. The tenth Stuart king of Scotland and the second Stuart king of England and Ireland (reigned 1625-1649). The second son of James I and Princess Anne of Denmark, he was the only king in British history to be publicly executed, and the first monarch to be publicly executed in European history.
After Charles I was executed for treason, the monarchy collapsed and the French Republic was established. This period is also known as the Cromwellian Interregnum. Charles I's eldest son, Charles II, inherited the throne after his father's death and did not exercise power until the Restoration in 1660.
Baidu Encyclopedia - Charles I (King of Scotland, England and Ireland)