<p>Firstly, no one fixed Blue Diamond's words because there was no time to waste (though they wasted pretty much time after).
</p><p>Secondly, the Diamond did not use their aura when Steven cried. (or at least, that wasn't shown on screen, they could have done it without showing off or it happened in the flash)
</p><p>Thirdly, Steven became a monster because he thought about his "monstrous" actions, like killing someone. He appears older if he thinks older, and he appears as a monster if he thinks of himself like that.
</p><p>Another thing is that the big guy is called "Monster Steven" in the credits. (I can't upload the proof right now, but you can check yourselves)
</p><p>So I think it is just a transformation and not a corruption.
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