Was Satan created to accuse, or has he assumed that role in rebellion?

Lucifer's initial job was as a cherub that covereth :-

14 Thou art the anointed cherub that covereth; and I have set thee so: thou wast upon the holy mountain of God; thou hast walked up and down in the midst of the stones of fire.

15 Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.

16 By the multitude of thy merchandise they have filled the midst of thee with violence, and thou hast sinned: therefore I will cast thee as profane out of the mountain of God: and I will destroy thee, O covering cherub, from the midst of the stones of fire.

When you consider the Sanctuary, the Mercy Seat which represented the Throne of God was covered by the wings of two cherubs. This was the work of Lucifer before his fall.

He became the accuser of the brethren after the rebellion

Was Satan created to accuse, or has he assumed that role in rebellion?

There may be some confusion in the following verses as they transition from describing the King of Tyrus to Satan but the application to Satan seems appropriate;

Ezekiel 28:14-15 Thou art the anointed cherub that covereth; and I have set thee so: thou wast upon the holy mountain of God; thou hast walked up and down in the midst of the stones of fire. Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.

It would seem unlikely that accusation would be part of Satan’s initial purpose as it only has value when the judging person has no knowledge. Since God is omniscient, it would not be necessary for someone to have to inform him about any error.

The act of accusation by Satan is illustrated for us with regard to Job.

Job 1:8-11 And the LORD said unto Satan, Hast thou considered my servant Job, that there is none like him in the earth, a perfect and an upright man, one that feareth God, and escheweth evil? Then Satan answered the LORD, and said, Doth Job fear God for nought? Hast not thou made an hedge about him, and about his house, and about all that he hath on every side? thou hast blessed the work of his hands, and his substance is increased in the land. But put forth thine hand now, and touch all that he hath, and he will curse thee to thy face.

Here we see a demonstration of accusation in advance. In a way it is an accusation of God.

Since accusation is done not for the purpose of truth, but to smear, distort, impune, and tarnish one might see where it would result from a fallen state.

It is important to look at the context of Revelation 9:11. The Destroyer is released from the abyss by the star that fell from heaven.

And the fifth angel sounded, and I saw a star fall from heaven unto the earth: and to him was given the key of the bottomless pit.
-- Revelation 9:1

Given this, there can be no doubt that Satan is the one responsible for the release of the inhabitants of the pit, and that the pit is part of the earthly realm.

We are also shown the purpose of the abyss:

And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand. And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years, And cast him into the bottomless pit, and shut him up, and set a seal upon him, that he should deceive the nations no more, till the thousand years should be fulfilled: and after that he must be loosed a little season.
-- Revelation 20:1-3

The abyss is a place to which God confines those heavenly powers who are bent on destruction.

So, Revelation 9:11 depicts Satan being given the keys to set loose the powers of destruction from their place of confinement.

Now, harken back to the story of Job, where Satan responds to God's esteem for Job's diligent practice of his religion:

Hast not thou made an hedge about him, and about his house, and about all that he hath on every side? thou hast blessed the work of his hands, and his substance is increased in the land.
-- Job 1:10

The only characters in the story of Job who are aware of this "hedge", are God and Satan. God knows about it because He put it there, and Satan knows about it because he has been confined to the region outside of its bounds (on all sides), which has hitherto kept Job beyond his reach.

Contrary to the understanding of some, Job, like all men, sinned. However, because his sin is not related to adherence to the Law, which we know was impeccable, it often goes unnoticed. Job's sin was related to the fact that he'd forgotten the "purpose" of his Law keeping, which is to advertise God's goodness and bring glory to Him.

Now, in order to give Job the opportunity to realise and repent of his sin, God needed to allow access through the hedge, to the powers of destruction. So, He gave the "keys" to the hedge to Satan.

And the LORD said unto Satan, Behold, all that he hath is in thy power; only upon himself put not forth thine hand. So Satan went forth from the presence of the LORD.
-- Job 1:12

The scenario depicted in Revelation 9 is the same scenario depicted here in Job, but on a global scale.

Was Satan created to accuse?

There is nothing in scripture to support the idea that God made automatons of any of His creatures. Rather, the clear vibe of scripture is that God gave all his created beings the same freedom to do as they please as He, Himself, possesses. This is strongly supported by the fact that both angels and man have the capacity for sin, i.e. to choose whether or not it pleases them to act in concord or discord with the commandments of God. Peter tells us that their sin was the reason the angels were cast from heaven (2 Peter 2:4).

No, Satan was not created to accuse. He was created with the freedom to choose his own path, which Jesus tells us (Matthew 25:41) will ultimately lead him and other like minded souls, to eternal separation from God, in Hell.