This scene was actually so sad and I want to ramble about it. Though Nero is talented, he was still a kid here. He didn't have much of a handle on his demonic abilities because he didn't know he had them until a short while ago. And then he gets literally crucified for having demonic abilities-- something he'd been afraid of since the religious cult he was raised in preached death to all demons (lol)-- only to be saved by the Yamato (unknowingly the same way his father was also saved by the sword as a kid). And after he temporarily takes out his captor, Nero falls to his knees and frantically puts his human hand over his heart and chest like he's trying to see if he's still alive, if his heart is still beating after having been nearly killed and severally injured. And given that he knows so little about his abilities, & based on how the scene is described in the novel, it seems like he thought he was *dying*.
But the worst part is when he just sits there rocking himself back and forth laughing, but it sounds like he's going to cry. But he has to push himself to continue on anyway because there is no one to protect him now. Credo sold him out and no one is coming to his rescue so he has to end this himself to save Kyrie.