They kill vandal savage and he taunts them that he'll just come back and while he's prone and incapacitated they just... Leave him there. They have the knife to kill him for good, they just need the right person to do it but they're sick. So.... why not just keep him incapacitated until she can do it?? Like, if he starts reviving again just stay there and smash his head in a little or stab his heart. Or, just pick him up and toss him into the sun or deposit him onto the moon. Like these are all perfectly viable things to do. And instead the show just... forgets and leaves him there. It would be one thing if they created a one or two line excuse to have wave it away but they don't. That was probably the worst, most glaring plot hole I've ever seen. Ever.