Complex sentence: Dixon is vexed by Mildred's lack of respect for his authority, and counters by threatening Red, who rented the billboards to her, and arresting her friend and coworker Denise on trivial marijuana-possession charges.
Split into simpler sentences: Dixon is vexed by Mildred's lack of respect for his authority, and counters by threatening Red, who rented her the billboards. He even arrests her friend and coworker, Denise, on trivial marijuana-possession charges.