Former mixed martial artist Lumumba Sayers -- who once fought ex-UFC star Derek Brunson inside a cage -- was arrested for murder over the weekend ... after he allegedly shot and killed a man to avenge his son's death.
The Commerce City Police Department says the incident happened at around 5 PM on Saturday during a children's birthday party at a Commerce City, Colo. park.
According to the arrest affidavit, obtained by Denver7, Sayers is accused of walking up to 28-year-old Malcolm Watson and shooting him in the head.