He knows what she did with Sasha/Daisy. He knows she showed up at his mom’s house in a wedding gown, hysterical, mascara running, demanding to see her kids and wouldn’t calm down or back down, even when Carly put her in front of a mirror to show her how crazy she looked. He knows she walked right through a court order on multiple occasions, even though that jeopardized her custody case. He knows she is generally unstable and possibly a danger to their children.
I’m not discounting the idea that Michael is angry and wants revenge on some level, but I honestly don’t think that’s his chief motive in keeping Willow away. He knows she can’t be trusted.
And Drew knows now, lol. 😁