Final answer:
Charles I was beheaded because he committed adultery and the parliamentary "roundheads" captured him.
Step-by-step explanation:
Charles I was beheaded because he committed adultery and the parliamentary "roundheads" captured him. Although religion played a significant role in the conflicts of the time, it was not the main reason for his execution. Charles I's downfall resulted from his clashes with Parliament and his attempt to rule without their consent.
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