He was born as a slave and manipulated into being involved in the genocide of his own race, then forced to live with the burden of having an immortal body filled with the crying souls of the thousands of people who had died.
He devoted at least a couple of centuries conversing with over 500,000 souls, getting to know each and every one of them, befriending them, and cooperating with them.
Hohenheim did not leave his children alone. He left them in the (more than capable) hands of Trisha, where he knew they would be raised well. He trusted that Trisha would be able to explain his absence to Ed & Al without revealing the truth about his body. He couldn’t have anticipated her death, and as he was travelling the entire country he had no way of keeping in contact with Trisha to check that she and their sons were doing ok (this is assuming the Elrics didn’t have a telephone).
Hohenheim had difficulty interacting with people, even his own family. He would probably have a very hard time writing letters back home, or talking to Trisha on the phone. He more than likely thought it best to keep his distance 100% until he could return home for fear of ruining any of his relationships with his family.
Hohenheim was immortal and had already lived for hundreds of years. A year or two probably wouldn’t have seemed like a long time to somebody with such a huge lifespan. Hohenheim likely still pictured his sons as young children, even after 10 years of not seeing them.
When Hohenheim returned to Resembool he expected to be greeted by the love of his life and his two young boys standing at the doorway to his family home. Instead he found a pile of burnt rubble, Trisha’s grave, Edward’s automail, and Alphonse’s empty body.
He completely blamed himself for Ed and Al’s attempt at human transmutation and subsequent injuries. He understood that they were lonely, grieving children and in his eyes the fault was all down to him and - taboo or not - the brothers had not done anything wrong.
Despite being the most powerful character in the FMA universe, Hohenheim was a pacifist and did not fight or injure a single person in the whole series.
He always put everyone else before himself.
The things which made him happiest in the whole world were all to do with his children. Alphonse trusting him, Ed and Al helping him on the promised day, Ed calling him Dad for the first time, getting to shake Al’s hand after recovering his body… his whole life revolved around his sons as soon as he met up with them again. He was willing to sacrifice himself for Ed and Al’s sake.
He died happy because he was so proud of Ed and Al and he knew that they had made incredible friends who would look after them for the rest of their lives.
Without him everyone would be dead.
All he wanted was a normal life.
Van Hohenheim was not a bad person.
Van Hohenheim was not a bad father.
He is romantically cheesy as hell
He wears glasses just so he looks slightly different to Father
He used to fly into an Ed-like rage whenever anyone called him unintelligent
The first thing he said to Alphonse after reuniting with him was, “My vintage armor!!”
One of the first things he said to Edward after reuniting with him was, “we have the same hairstyle”.
To everyone who keeps on including Hohenheim in the "Bad Dad" trio with Gendo Ikari and Grisha Yeager:
And remember this:
And let’s not forget the reason WHY he left:
If an absolute nut-case was planning on sacrificing an entire nation of people to absorb God and rule, would you just dick around with your children, let it happen and let them die, or would you sacrifice the happiness you have wanted for so long in order to find a way to stop those plans and implement that plan? Which one would have been more ass-holish?