They don't want the same thing, they ARE the same thing (as Streiber's so-called aliens are demonic)!
Bad angels maybe fabricasting/growing clones into which the demonics are inhabiting. If one of the meat puppets is killed the demonic merely moves to another carrier.
These "demons" are subtler and much more clever than those portrayed in "possession" movies. If these are demons, they are different than the ones we have come to know as causing people to hurt themselves, curse uncontrollably, spit nails, etc.