I'm betting it's something physical but invisible kinda like the arms in Elfin Lied. At first it seemed like some kind of psychic force since people's bodies just kinda blow up in places, but in this chapter we saw the old guy's arms twisting and stuff before they blew. That and there seems to be a proximity factor involved.
Either way, hopefully they either reveal what power is being used or they need to just have it die. It's kind of annoying to me to have a monster so cheap that it can just kill anyone it wants regardless of strength at its leisure.