I honestly liked him at one point but sadly it was all a facade :( His current behaviour is just a means to obviously keep the mc there and watch everything unfold.
Spoilers again btw
The bet between the duke and demon king was to see whether he'll survive the 8 year long war or how long he would last. He survived it and so they used him for their own gains. Btw the duke, demon king and king were all aware of mc's actions. So they knew he was a spy for both the demon king and king (i'm sure everyone is aware of this tho?)
There were many other spoilers but I sadly don't remember much besides the main ending.
His childhood is built on misunderstandings because of the lack of communication and reassurance from his family.
The memories that the mc has of his family are misinterpreted. In a situation where mc is always sick and alone with rumours spreading, there is no doubt that what his family members say to him will be misinterpreted. Due to having a weak constitution, the mc is always kept indoors or is bedridden. Many will know the reason as to why the mc is never present for events or outings however, there will be people who will doubt and question this, thus leading to the discussions and rumours between the maids and many other people. Rumours are inevitable and will spark for many reasons but they can be addressed. Measures should have been taken to prevent these rumours in their household whereas the rumours from the public are often uncontrollable or will always raise doubt, this is where his parents step in.
Yes, they were unaware of how this would affect the mc and didn't notice the signs that it was already affecting him negatively, however his negative thoughts wouldn't have grown this large had his parents reassured their love for him and explained their actions and words thoroughly. Small conversations with him every now and then would have eased his doubts as well as asking questions when either the parents, brother or mc are unsure of things.
Now the 8 year war as most have seen it was indeed fabricated by the duke. From the first to last memories he had of his family, everything was a misunderstanding.
(This is a VERY short summary of the ending)
After Cruel's (his brother) sacrifice, Dion realised the truth and his split personalities fused into one. He sought out the ones behind his demise which were the king, duke and demon king and killed them (The duke and demon king are killed because they were the reason behind the 8 year war (it was a bet) the king also died because he watched everything unfold whilst continuing to use mc). Dion continued to live on for a few years in order to respect Cruel's wishes and then kills himself as everything has been accomplished. In the after life Dion wishes to bear Cruel's karma (bad deeds) where he will suffer miserably in the afterlife as well as when he reincarnates into his upcoming life. However there is a price for wanting to bear the massive amount of karma which is to give up his eyes. In Dion's upcoming lives he will be blind and suffer through them. Dion happily gives them up because he wants Cruel to rest in peace and live a happy upcoming life. Ed, Dion's assistant buried Dion next to Cruel's grave.
I do believe that after a certain amount of time, Dion will accumulate a decent amount of good karma and be reincarnated into a life where he has his eyes and can be happy. It might also be a life where he is once again brothers with Cruel ╥﹏╥