Addressing the OP only, not saying that you're right or wrong, but consider this. Evil has always dwelt with Good, just God back to the Garden of Eden.
but also consider this. Evil is not SIN, let me say this again, Evil is not Sin, but can cause one to sin.
also, consider this. the term demons are another idenification of evil attributes, or characteristics. let's give a good example of this.
Mark 5:1 "And they came over unto the other side of the sea, into the country of the Gadarenes.
Mark 5:2 "And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit,
Mark 5:3 "Who had his dwelling among the tombs; and no man could bind him, no, not with chains:
Mark 5:4 "Because that he had been often bound with fetters and chains, and the chains had been plucked asunder by him, and the fetters broken in pieces: neither could any man tame him.
Mark 5:5 "And always, night and day, he was in the mountains, and in the tombs, crying, and cutting himself with stones.
Mark 5:6 "But when he saw Jesus afar off, he ran and worshipped him,
Mark 5:7 "And cried with a loud voice, and said, What have I to do with thee, Jesus, thou Son of the most high God? I adjure thee by God, that thou torment me not.
Mark 5:8 "For he said unto him, Come out of the man, thou unclean spirit.
Mark 5:9 "And he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many.
Mark 5:10 "And he besought him much that he would not send them away out of the country.
Mark 5:11 "Now there was there nigh unto the mountains a great herd of swine feeding.
Mark 5:12 "And all the devils besought him, saying, Send us into the swine, that we may enter into them.
ok, from reading these verses, it appears that there are many "devils" ... right NO, only "ONE". notice in verse 2 "spirit", no s at the end to indicate more than one. it was one spirit that had many "attributes" of evil. but notice when the spirit answered he said that he was "Legion" meaning MANY, but it was one spirit. just as it it said in verse 12, "Devils", with the "s" at the end. so are there many devils or one?.... of cource one devil. but he come in "many" forms. lust, fornication, adultery, envy... ect.......
yes, we possess those characteristics, or traits, even while saved. these are the works of the flesh. again, evil is not sin, but can cause one to sin. 1 John 1:8 "If we say that we have no sin, we deceive ourselves, and the truth is not in us". but here's the kicker, the Holy Spirit if you let him will help keep you from sinning. just as the very next chapter states, if we fail. 1 John 2:1 "My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous".
always understand the Spirit helps our spirit, not our flesh. Galatians 5:16 "This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
Galatians 5:17 "For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
Galatians 5:18 "But if ye be led of the Spirit, ye are not under the law.
you can finish the chapter here in Galatians,
we Hope this Helps