Ok, so let's run this scenario.
Bad guy says forget this and gets up and starts walking away. Good citizen orders halt but he doesn't comply.
Citizen shoots and kills bad guy.
What happens now?
Citizen takes knife out of pocket, wipes off prints, puts knife in BG's hand. Problem solved. About the same as planting drugs during botched raids or traffic stops.
Kidding of course. I'm sort of surprised that didn't happen (BG get up and walk away). Personally, once he's on the ground I'm on him anyway until someone else comes to assist to prevent that from happening. Then the quick cuffs come out of my bag.