Its called common sense.
Imagine that you were a very good child and took care of your parents in old age. You worked hard to support the family and provided everything they needed.
Your brother was unable to provide any meaningful assistance to the family. However, upon death your parents left everything to your good-for-nothing brother. How would you feel?
...um, I'd feel like my parents made the right choice because if my brother is so untalented that he can't even offer any meaningful assistance to my family, then he needs whatever my parents left him, while I, being talented, wouldn't need that stuff.
Plus, your example has almost nothing to do with indra and asura. Asura gained the ability to 'provide meaningful assistance to the family' before he was given hagoromo's gift.
Next, if I was so insecure and obsessed with power/money that I would go to war against my own brother because my father gave him the wealth, then it just proves that my father was right in not choosing me with the task of finding world peace.
Indra felt betrayed by Hagoromo. He was always more naturally gifted and intelligent than Ashura but Hagoromo stilled favoured Ashura.
Black zetsu just took advantage of Indra's already existing hatred for Hagoromo and Ashura.
Even Hagoromo admits it was his fault. Stop playing dumb.
Unless you believe and want to assert that Indra has no free will and only hagoromo does, the indra is responsible for indra's actions, not anyone else.
It's indra's fault for feeling betrayed by hagoromo. Hagoromo never promised him anything, it's indra's own fault for creating these false expectations and getting mad when these false expectations weren't met. It's also indra's fault for choosing to act upon his hatred for hagoromo and asura, and for agreeing to do whatever black zetsu urged him on to do.
Hagoromo's goal was world peace. Indra proved hagoromo's decision to choose asura right when he decided to war against asura. If Indra was truly after world peace, then he puts whatever hatred he has aside for that goal, he doesn't do the exact opposite of that goal just because he's jealous of his brother.
You are the one who's playing dumb, it's blatantly obvious when you have to constantly assert that Indra is not responsible for indra's actions.