These angels are the elect angels.
They were created sinful just as man was, but the mystery is how they were able not to sin as opposed to the angels that did sin.
With regard to man who was created sinful and did sin because the truth is that anyone who is holy cannot sin as the last Adam proved.
Sin comes from sinners. Sin does not come from holy.
Thus, anything and everyone created outside of God will have to be sinful because Scripture states that there is only ONE God, there is NONE like Him and He gives His glory to NO ONE (Isaiah.)
There is only ONE Person that can stand before a Holy God and that is a Holy Son.
Scripture states unequivocally that the angels that sinned are locked up.
4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment; 2 Peter 2:4.
How do you account for sin in the world if the angels that sinned are locked up?
Lucifer/Satan and the pride that was found in him when God placed him in the garden of Eden before the creation of man.
Isa 14:12 How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations!
Isa 14:13 For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:
Isa 14:14 I will ascend above the heights of the clouds; I will be like the most High.
Isa 14:15 Yet thou shalt be brought down to hell, to the sides of the pit.
2 Peter 2:4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
Jude 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
A little background first to show these angels that are in chains of darkness are separate from the third that followed after Satan as they too rebelled against God leaving their first estate that was their proper place in heaven. These chained angels can not be the same angels as the principalities and power of darkness still in high places under the direction of Satan, Ephesians 6:12.
1 Peter 3:18-19 Jesus never went down to hell (lower parts of the earth) to preach as scripture says He went to preach to the spirits in prison. When Jesus laid in the tomb for three days his body was dead, but His spirit was very much alive. In 1 Peter 3:19 it says Jesus being put to death in the flesh, but quickened by the Spirit went and preached to the spirits in prison. The spirits in prison that Jesus went to preach to are those angels that are reserved in everlasting chains under darkness unto the judgment of the great day. Scripture does not say what Jesus preached to them, but some speculate that He was proclaiming to these fallen angels that Satan had no victory over His death. These angels are separate from the third Satan gathered with him as these angels bound in chains kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day, Jude 1:5-7. These angels were disobedient when once the longsuffering of God waited in the days of Noah while the ark was being built, 1 Peter 3:18-19. Big difference between these chained angels and the other ones we read about in Ephesians 6:12. Notice where it says they are reserved in chains meaning they are bound by God and can no longer cause man to sin.
These other angels being a third that oppose God under the leadership of Satan as being his messengers here on earth as they too are created spirits of God that is why God could never destroy any of them because a spirit can not die. (Matthew 25:41; 2 Peter 2:4; Jude 1: 6; Ephesians 6:12). We often call these types of angels demons/devils/evil spirits/unclean spirits for which an everlasting fire is prepared by God for Satan and these angels, Matthew 25:41.
The term demon comes from the Greek "daimon". Demons (fallen angels) are also called evil spirits, unclean spirits and devils in scripture as they are disembodied spirits as angels have no form. The use of the terminology of demons, evil spirits, unclean spirits and devils are interchangeable in scripture as they all mean the same thing. They exist in a demonic realm as principalities, Romans 8:38, 39; Ephesians 6:12, and have a hierarchy of rulership controlled by Satan as being his messengers, Matthew 12:22-30; Mark 3:22; Luke 4:35. They have no physical form, but can viciously posses a body, Matthew 8:16-31; 12:43-45. Demons are numerous in numbering and can also speak like the Legion (2000) of unclean spirits in Mark 5:9.