They didn't kill Garth until he registered "Hope." They could have easily done so up to that moment. And Tula said that was what she'd been waiting for. Also, the soldiers who were in the plane with the Batman villains who were killed--like with bullets--by the rings were not revived. So it appears that the target has to be "ripe" in that way before plucking the heart.

Though really, if one of these fuckers is coming at you, you're gonna be feeling SOMEthing.

But the whole point of their talking to their prey before killing seems to be to make sure the emotion rises. Which is why they talk like angry versions of themselves. Except the clue in all this that Nekron, not they, are talking is this: DOLPHIN COULDN'T TALK. Yet she says plenty, even with most of her head gone, during the Mercy Reef attack.