I believe that Judas betrayed Jesus because he believed the devil's lies about Jesus not being the Son of God and God the Son.

Judas was already stealing from Jesus' financial resources long before the Bible says he had thoughts of betrayal. He was a petty thief:

John 12:6 (RSV) This he said, not that he cared for the poor but because he was a thief, and as he had the money box he used to take what was put into it.

The devil exploits relatively lesser sins and leads one to commit greater sins. Thus Judas eventually he takes money to betray the Son of God. But how can such a monstrous evil (exponentially greater than stealing from the pot) be explained? It can't; it's an insane act of pure evil. The Bible says that the devil entered into him (as the sad end result of being taken in by Satan's ploys) and then he went and betrayed Jesus for money:

Luke 22:3-4 Then Satan entered into Judas called Iscariot, who was of the number of the twelve; [4] he went away and conferred with the chief priests and officers how he might betray him to them. (cf. Mt 26:14-16; Mk 14:10-11; Jn 13:27)

John 13:2, 26-27, 30 And during supper, when the devil had already put it into the heart of Judas Iscariot, Simon's son, to betray him, . . . [26] . . . So when he had dipped the morsel, he gave it to Judas, the son of Simon Iscariot. [27] Then after the morsel, Satan entered into him. Jesus said to him, “What you are going to do, do quickly.” . . . [30] So, after receiving the morsel, he immediately went out; and it was night.

Now, possibly, some of the (and maybe the initial) reason for Judas' lack of faith in Jesus was a lack of belief in the Eucharist (see 1 Cor 11:26-30). This was the context of John 6:70. “Many of his disciples” (Jn 6:60) were expressing doubt about the Real Presence in the Eucharist, after His extended discourse about that very thing in John 6. And after that, “many of his disciples drew back and no longer went about with him” (6:66). This is the only Bible passage in which “disciples” are described as having forsaken Jesus once and for all.

Then the text states the following:

John 6:67-71 Jesus said to the twelve, “Do you also wish to go away?” [68] Simon Peter answered him, “Lord, to whom shall we go? You have the words of eternal life; [69] and we have believed, and have come to know, that you are the Holy One of God.” [70] Jesus answered them, “Did I not choose you, the twelve, and one of you is a devil?” [71] He spoke of Judas the son of Simon Iscariot, for he, one of the twelve, was to betray him.

This ties in the theme of whether Judas believed that Jesus was the Messiah and God the Son, and the Eucharist. It may have been a reason why Judas turned against Jesus, allowing the devil to get a foothold in his soul. Some followers left because of the Eucharist. The twelve (excepting Judas) did not, because they believed He was God. Unbelief is of a piece, in other words: if you disbelieve Jesus regarding the Eucharist, chances are you will also regarding Who He is.

If He didn't believe in the Real Presence, then the devil would have had an influence on him at that moment of lack of faith, during the Last Supper (Jn 13:27) and so entered into him all the more. He went out and betrayed Jesus immediately after.

If we tie this all together (in my own speculative “theory”): there is the profit motive, the possession by Satan, and disbelief in the Eucharist and in Jesus as the Son of God. The latter, if true, could possibly explain to some extent, why he felt led to betray Him, because if in fact Jesus was not God Incarnate (if Judas came to believe that), then he would regard Him as a liar and blasphemer of the worst sort: precisely as His enemies among the scribes and Pharisees thought.

Therefore, He was only fit to be executed under Mosaic Law, in which case it was right and just to participate in handing Him over to the officials for a trial on the grounds of blasphemy and being a false Messiah, with the promise of money, too, as an extra bonus for a thief.

Several Church fathers (including St. Augustine), also made a connection between the Eucharist and his betrayal, as I have done.