So if the angels are immortal, and Satan and the demons were once faithful angels, how can they be killed....WTS reasoning.
Insight Book Volume 1--Immortality
This grant of immortality to the Kingdom heirs is all the more remarkable, in view of the fact that even God’s angels are shown to be mortal, despite their possessing spirit bodies, not carnal ones. Angelic mortality is evident in view of the judgment of death entered against the spirit son who became God’s Adversary, or Satan, and also against those other angels who followed that satanic course and “did not keep their original position but forsook their own proper dwelling place.” (Jude 6; Mt 25:41; Re 20:10, 14) So the grant of “indestructible life” (Heb 7:16) or “indissoluble life” to those Christians who gain the privilege of reigning with God’s Son in the heavenly Kingdom marvelously demonstrates God’s confidence in them.—See HEAVEN (The way to heavenly life); LIFE.