Oh Harald you should have listened to Webec all along . What a team he and Rocks would have been .
Imu played him for his foolishness .
See, in many instances a flaw in personality would be more negative. Like being arrogant, hot headed, selfish. Things you actually can attribute to being a flaw.
Harald's flaw was simply being too soft-hearted in a time Elbaf needed change. It shows that yes, many examples of One Piece having evil kings but Harald, despite not being evil, is a good man but a terrible King. I think Oda is trying to display that yes, you should resist tyrants, but being too good hearted and selfless is also a trait of a bad ruler.
You cannot throw away your culture, you cannot just weaken your country in the hopes for peace.
And while I understand he does this also for Rocks and Ida, he fails to realize what they would want.
He is in a cage with no bars but it is cage. One he built himself.
So overall, Harald. Absolutely good hearted man who only wanted the best for Elbaf. Who wanted to honour the sacrifices made and lives lost for the grand cause but a terrible king.