Paul writes in 1 Corinthians that the rulers of this age killed Christ. Some say that he meant the spirit demons. In Ephesians 6:12 he calls the spirits by a similar phrase- “the rulers of the darkness of this age”. Did Paul then believe Jesus was killed in the spirit realm by demons and not as the Gospels say, on earth by Roman authorities?
! Cor 2:6-8 was not speaking of a demonic spirit realm. Keep in mind the first audience.
An age is a time period. Too often the Greek word "age" is incorrectly translated as "world", and it is best to think of it in the terms of "their world as they knew it". Their day, their time, their culture, their century.
The book of 1 Corinthians was written approx. 65-66 AD. (1) The rulers of this age when the book was written in the 1st century AD were those of the "council", the Sanhedrin, who in turn were answerable to the authorities and governors of Rome under Caesar.
At that time, the rulers who crucified our Lord and Savior were Caiaphas and those of the Sanhedrin who held an illegal trial against all the Mosaic Law, and falsely accused Him of blasphemy (Matt. 26:57), and who then conspired to turn Christ over to the Roman governor Pilate under charges of treason and sedition in order to try to keep their people from revolting. By making the claim to Pilate that Jesus was calling Himself the King of the Jews, the charge became a political issue against Caesar and the Roman governor would oversee punishment.
All of these rulers, from the Sanhedrin to Caesar himself were the "rulers of this age" (YLT) or the "princes of this world" (KJV) who were involved in the death of our Lord. Jesus told Pilate,
"Jesus answered, Thou couldest have no power at all against me, except it were given thee from above: therefore he that delivered me unto thee hath the greater sin." (John 19:11, KJV)
Please note that the definition of the Greek "diabolos" (English devil), Strong's Gr. 1228 is one who accuses falsely, a slanderer. (2)
Therefore the rulers of this age, and the princes of this world who delivered Christ up for crucifixion were men involved in slander, false accusations, and murder - devils. But, that does not make them "demons" of the spirit realm. They were taking a stand against God, which made them adversaries - ha satan.
For those interested, my post "The God of This Age, And The Prince of The Power of The Air" provides more scriptural proofs. (3)
1) Dating the New Testament here
2) Strong's Gr. 1228 diabolos - Biblehub
3) The God of This Age.... - ShreddingTheVeil
1 Corinthians 2:6-8
Here is what I get from the Greek:
6But, we are talking of the wisdom of perfect things ‒ wisdom, though, not of this age, nor of the rulers of this age, about things that will pass away.
7 No, we are talking of the wisdom of God! About a mystery ‒ hidden wisdom ordained by God before the ages, for our glory. 8 Wisdom none of the rulers of this age has understood. For had they understood, by no means would they have crucified the Lord of glory.
In verse 6, Paul is contrasting wisdom of "perfect things" (τοῖς τελείοις) with wisdom of "passing away things" (τῶν καταργουμένων), as he did in 1 Corinthians 13:10, i.e. "that which is perfect" (τὸ τέλειον) with "that which is ... done away" (τὸ ... καταργηθήσεται)
But when that which is perfect τὸ τέλειον is come, then that which isτὸ in part shall be done away καταργηθήσεται*.
-- 1 Corinthians 13:10 (KJV)
In regard to who these "rulers" might be, Paul here uses ἀρχόντων (from ἄρχων : Strong's G758 - archōn), which is the standard Greek word for rulers/princes of this world, whereas in Ephesians 6:12 (which the OP cites), he uses the word κοσμοκράτορας (from κοσμοκράτωρ: Strong's G2888 - kosmokratōr).
κοσμοκράτωρ only occurs once in the New Testament, in Ephesians 6:12. If Paul wanted to inform his readers that the rulers in 1 Corinthians 2:6 were "spiritual beings in heavenly places", then he would have used κοσμοκράτωρ rather than ἄρχων.
In 1 Corinthians 2:6-8, Paul is talking about the rulers of the people, i.e. those to whom Pilate spoke:
13And Pilate, when he had called together the chief priests and the rulers τοὺς ἄρχοντας and the people,
14Said unto them, Ye have brought this man unto me, as one that perverteth the people: and, behold, I, having examined him before you, have found no fault in this man touching those things whereof ye accuse him:
-- Luke 23:13-14 (KJV)
The very ones who later stood taunting Jesus as he hung on the cross:
35And the people stood beholding. And the rulers οἱ ἄρχοντες also with them derided him, saying, He saved others; let him save himself, if he be Christ, the chosen of God.
-- Luke 23:35 (KJV)