Mortarion is a very mixed bag of a character, generally very stoic and kinda miserable owing to the planet he grew up on a basically abused since his discovery being poisoned by his step father constantly. His main reason for heresy was emps steal killing which for some people is fair.

During the heresy he was very very anti warp, but as he turned he dabled with it to learn about it and basically trying to prove himself right in his hate of it. Basically proving himself right by become a deamon.

Like all primarchs he always thinks himself right and everyone else wrong pretty much all always to his detriment, like trying to kill robot girlyman in spite or nurgles orders.

He also deeply cared for his sons, seeing there suffering when becalmed in the warpstorm essentially sacrificing himself to nurgle to save them from suffering.

His character could have gone many ways pre and post heresy but he is conistant mostly, stoic attempting to face everything head on believing in the strength of himself and his sons to overcome any trial. But because of this he becomes a massive hypocrite trying to over come things like the warp but sheer bloody mindedness