He was basically ruling like a tyrant. Not an evil psychopath tyrant but he was ignoring and then banned parliament, taking unfair taxes and all of that. Then there was his marriage, trying first to marry a spanish princess, then he married a french one. He and his wife were catholic, not protestant. So yeah, he was overthrown in the civil war, arrested, and then comes his trial, he refuses to plead, stating devine right and that the court can't try a monarch, and looks as though he's winning the court case when they sentence him anyway.