It’s funny to think about the point of each of the prequels individually and if they actually add to the Skywalker saga.
TPM right out of the gate falls into irrelevance by depicting Anakin before he is old enough to have developed any moral nuance. We learn nothing about him except that he’s a really swell little kid who flies good and wants to do other things good too, knowing nothing of greed. Oh, and he also has a superficial crush on the queen. Virtually every experience gained in this movie is ignored by its sequels.
AOTC decides to go into developing this superficial crush angle and turn it into True Love, which fails epically. But not to worry, we still have Anakin as a good man nobly upholding the Jedi ideals and resisting the call of the Dark Side. Except that he murders a tribe of sentient creatures (people), thus torpedoing any attempt at the ‘good man’ angle. But maybe if we discard these two movies, the last one will prove that he was still maturing and that he will become a good man, and maybe give some compelling reason for his fall.
So anyway Anakin straight up beheads an unarmed prisoner in the first twenty minutes of ROTS. This somehow isn’t in the scene where he falls to the Dark Side. No, in the scene where he falls to the Dark Side it is because of True Love, Unlimited Power, or Jedi Are Evil maybe. True Love rings false because he strangles his True Love anyway, and The Jedi Are Evil angle feels like petty childishness coming on the heels of his denial of Mastership. So we’re left with Unlimited Power, essentially simple greed. This is the least compelling answer of all, but has thematic roots in TPM so I guess that’s the answer.
There is one interesting aspect of all three of these explanations for his turn to evil - they are all thematically established in the first installment in the most superficial way possible due to Anakin’s immaturity, and then all three are grossly mishandled in the second installment to such an extent that it is manifestly clear that they are all subservient to, and governed by, Anakin’s persisting immaturity. This overarching theme, Anakin’s immaturity, is the only reason for his fall to the Dark Side that rings true.
And this destroys the message of the OT.