He did it with the deliberate goal of taking Buddha’s followers & putting himself at the top. In other words, he didn’t have a genuine disagreement.

he wanted to outshine Buddha’s rules by cooking-up some new “requirements” that he knew weren’t genuine but still seemed “more ascetic & impressive”, only presented in order to make Buddha’s followers leave their sangha and join his.

If Devadatta had just had a disagreement, then left, he wouldn’t have been cast as a villain.

Not many people know that the “schism” rule only applies to people who present fake and dishonest misrepresentations for the sake of derailing others from their awakening & making them give you donations and followers.

It’s the same thing with the “rule against claiming attainments,” it only applies to people who knowingly lie about attainments for the sake of gaining followers and donations, and doesn’t apply to people who are simply mistaken about their progress.

Of course liars have evolved — once they realized that there’s a reduced punishment for mistaken & disagreeable people, liars always say “I was just mistaken about my attainments” or “I didn’t want a schism I simply had a genuine disagreement.”

That’s why the new cultural practice is to avoid all claims to attainment & all schismatic activity. Because liars have figured out ways to look the same as mistaken innocent people. Therefore nobody wants to even risk the mistake, since that puts them in the same bucket as liars who want to seem mistaken.