Either of the suspects in the two articles above are possible - an armed robbery gone bad or a crazy guy with a violent hair-trigger temper who chases people and threatens harm or carries it out.
The robbery would be bold in daylight during a busy time of day, but it sounds like its happened before in that spot.
The crazy guy is also possible. He previously was involved in a road rage incident where he chased down a car with a husband, wife and kids inside and threatened to shoot them. The West Park area is close to the park where Sledge and Brown were murdered.
It's possible either Sledge or Brown did something to anger this guy while driving to meet each other at the Metropark. Hurt could have been at the park that day, saw them and decided he didn't like them. That's how he killed his neighbor who was out mowing his lawn.
If a robbery, its possible Carnell had gotten up from the bench to stop the guys, possibly meaning they had approached from behind and gave orders to the couple. IIRC, he was found on the ground near the bench as if he had stood up.
I hope both of these incidents help the FBI and police bring some closure to this case.