He might have been a “rotten excuse of a father” but if he hadn’t left Amestris would have been destroyed.
"You’ve certainly mellowed out… you used to be fun, full of life and emotion. Lust, Greed, Sloth, Gluttony, Envy, Wrath, and Pride. Of course, excessive want will destroy anyone, but those same desires are necessary to understand what it means to be human." Van Hohenheim, FMAB
This was the best quote in Fullmetal Alchemist Brotherhood and I will hold this true to my heart. There was always a deeper meaning to Fullmetal Alchemist Brotherhood and theres so much to learn from it. Thank you Hohenheim.