SPOILER: all elves are just ridealongs to the demons that are functionally the means for resurrection. the 'forbidden' magic only works by fusing an elf to a demon and then implanting the demon in your target. In the end they all go off the rails and surrender control to the demons. The third eye is each of them their elf self, but the corresponding demon of a similar wavelength comes out when the eye is open. they're moooostly resurrected prior to the big climax, but are 'perfectly' resurrected now.
"going nuclear" happens as a failsafe behavior that can also be triggered at will. The other elfs are 'perfect' resurrections, and subject only to demonic overtaking during extreme moments.
I'm unconvinced they did not have this habit before the resurrection though. It's probably why people hated them so much. They're basically angels that can fall real real far.