mutsuki, you idiot! i told you not to look this way!
h-hurry up and run-- hyaan!
...say, mutsuki-kun, you know how to play rock-paper-scissors, right? it's the most-practiced form of battle in this world... and this works the same way.
the flames of the angels are a purifying light, and thus we demons cannot beat them ourselves.
but angels are unable to interfere with the products of creation itself --that is, beings descended from those who ate from the tree of knowledge. that means that you humans are untouchable for them.
on the other hand, fetus is limited to the use of human knowledge, and thus is unable to kill those from beyond your realm --us demons.