>>95941543
And that's because he got goaded by the Emperor, who was trying to humble him ("there are foes even you cannot slay alone" - 'The Lantern') instead of actually helping him ("There are foes you cannot slay alone. But you're not alone anymore" - 'My Ass').
The Emperor could've helped Mortarion instead of steal his kill, just like how He could have rescued Angron and his freedom fighters instead of kidnap him and left them all to die.
It doesn't even change anything or create an "alternate heresy": Mortarion would still dislike Him and psykers, believing they taint anything they touch. Angron would've kept his Butcher Nails friends and their taint would've contaminate the legion, and he'd still rebel because the Emperor is another tyrant who demands the stars dance to His tune.
He did allow Corax to finish his rebellion, after all.