World Eaters have had "good guys"- Angron killed most of them on Istvaan III, but there's a character in Battle for the Abyss - and the characters in The Outcast Dead were jailed on the first news of the Heresy- before they could decide. They were stationed on Terra.
I think the third Garro audiobook is supposed to have a "loyalist" World Eater.
So there's room to have a World Eater in that role.