Guilliman was once a hero of the imperium but no longer. He has consorted with xenos and heritics alike and has fallen away from the emperors light. He can no longer be trusted to lead the Imperium of Mankind.
Proof
He has been talking to the xenos ,Eldari, who even "resurrected him". As any self respecting man of faith knows this is impossible. He has likely been replaced with a puppet imposter to ruin the Imperium from the inside. We can not allow him to continue to rule or the imperium is doomed to fall to this xeno threat.