God created an angel we know as Satan (the adversary or accuser) who had a free will.  Satan chose to rebel against God, and ever since has been trying to destroy God's creation out of his hatred for Him.  God didn't bring sin into His creation, Satan did.

Just as God created mankind with a free will, He also created angels with a free will, and both had a time of probation in which they could totally choose to obey God or choose self above God.  Satan, who was the highest of the angels, chose self above God along with a third of the angels who followed him in his rebellion.

At that time, they were confirmed by God in their rebellion; they became fallen sinful angels for all eternity.  Those who chose God above self were also confirmed in their choice. They are the holy angels for all eternity.  The time of their probation has passed; now each is confirmed in their choice.  Holy angels are totally holy and cannot be otherwise; just as fallen sinful angels are what they are and cannot be otherwise.