I have a theory on why Mortarian fell, if indeed he was ever elevated. It comes in two parts:
1) Similar reason to Angron, his entire existence was focused on toppling the tyrants of Barbarus. He had destroyed, over-thrown and freed the humans from all other warlord. He believed he was invincible, able to breath what no human should be able to, achieving feats no human could, the pinnacle of a kind. The only remaining enemy was his 'father', the man who raised him*. When he reaches the mountain top, he is unable to do the deed he has been building to for years, Imagine being denied his final victory, nothing else is above him, save the one thing he wants, revenge against the oppressive dictator. Then, some stranger snatches his victory from him. There must be huge resentment from Mortarion at this figure for talking the thing he has wanted from the day he has escaped? Would he not feel resentment at not even being allowed to die and avenged, but knowing he is dying and having to watch a stranger take his goal from him.
2) Also, as a side note, consider what kind of a mental complex that must build, his surrogate father was an 'evil' overload, who was not a true human, who oppressed fellow humans whom Mortation felt sympathy for, and the Emperor is...almost exactly the same! The Emperor could be viewed as a tyrant, who exploits his species for his macro goals (and as many have pointed out) and not caring for the individual. Could Mortarion, having seen another person who claims his father and acts exactly as his previous 'father', sought another way to over-throw this new tyrant, and therefore attempted to kill this 'warlord' where he had failed last time.
Given that, even when the Heresy was about to start and was instigated by his closest brother Horus, and Typhus and Grulgor could see that Nathaniel Garro would never side with this rebellion, Mortarion gives Garro the benefit of the doubt, and gives him the chance to turn with his legion, showing once again that he is one of the most empathetic primarchs.
Also, shortly at the end, the only reason he turned to Nurgle was that his legion was afflicted but couldn't die, and he felt like once again he was on the hill-top of Barbarus, unable to fell his greatest adversary once again. The suffering of his legion, combined with his inability to defeat his most challenging opponents lead him to desperation for a solution, a solution which Nurgle had in his open hand (Similar to Magnus wishing to save his legion from the Flesh Change).
Just my thoughts anyway!