my friend allow me to offer you my insight. . .
you cannot kill demons. . . for one they very rarely leave the astral realm. . .
for two what makes you think that you can kill an immortal being. . . in order to "kill" something that something must first be alive. . . three nine times out of ten what you will deal with is not a demon, but a human soul. four demons do not inhabit human bodies or animal bodies for that fact. why would you want to put yourself in the body of a living being and be weakened by that creature when they can simply control said creature. five if you truly believe that you are killing demons then there is an amazing place called the psychiatric ward. . . maybe you should go and talk to them about your "problem" there is a difference between reality and fiction, you my friend simply need to find what is real and what is not.
May the stars guide you on your path and keep you in their light, may the moon show you the way to your destination may the wind blow at your back and keep you cool on your endless journey. have peace and tranquility through your days .until next time good night my companion under the gods.