The entire kingdom is so unappreciative istg, mc's been diligently protecting the kingdom since he was a kid but they treat him like he's the villain of the world. Also, you don't see any instances of the people of the kingdom suffering meaning he at least did a decent job of managing the kingdom. So his only crimes are declaring himself a demon lord, protecting the kingdom from countless invasions, and ruling with an iron fist (but not to the point where his subjects were suffering). Also, it doesn't even seem like he sends human soldiers to fight in the front lines considering he has the seven demon princes and the ability to create his own soldiers.
Now look at where the kingdom's ended up, being attacked by all sides even with the MC's help. Internal conflicts, an empress who can barely handle the crown, countless attempts at invasions, etcetc. Not a single word of thanks or realization, like "hey maybe demon King menas wasn't so bad, he protected us all this time". No, instead he got betrayed and branded as a villain, especially after everything that happened to him as a kid.
Mc is honestly a better person than me, I would've seriously turned into an actual villain a long time ago if I was put in his shoes.
Spoilers
No wonder the (ex) fiance isn't the ml, he actually apologized and listed out what he did wrong and said "I'm sorry". He said "I'm sorry", did I stumble into a different genre? You won't ever (or very rarely) find ml's saying "I'm sorry" and properly addressing their wrongs to the fl.
He's also respecting the fl's decision to break things off, even though in this ch (27) he wanted to reverse that decision. He's also still willing to help the fl even though he knows their engagement will be moot later on.
where did you read the spoiler?
From novel updates spoilers