You think part of why Arthur never gave Yellow a chance is because he believed he didn't have anything to do with John becoming "good" (-better/more empathetic) ?
That he thought it was all Lilly and that without his own absence, John could have never understood (/fallen in love with) humanity ?
Do you think he believes John changed in spite of his influence and not because of it ?
And so that would never happen with Yellow so he just had to be and remain evil.
Which self fulfilling prophecy cause what are you gonna do when you're being dismissed and compared to a oh so much better version of yourself and you can never get what he had whatever you do.

















