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Source: https://writers.stackexchange.com/a/40539
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/40539
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

What you're describing is a trope known as [Broken Pedestal](https://tvtropes.org/pmwiki/pmwiki.php/Main/BrokenPedestal) (tv tropes link). It describes the painful disillusionment with someone the MC considered a role-model, or otherwise a person to be respected and admired, until discovering that character's "true colours".

Such disillusionment can be a powerful motivation, and often a source of conflicting emotions: the MC might turn his back on his former hero, or he might wish to redeem him (while still actively opposing him). Consider, for example, what Luke Skywalker thought his father was, and his response upon learning that his father is Darth Vader.

As such, there is no need for an additional motivator, in the form of a friend etc. being killed. In fact, such an event would be "too much", it would dilute what you're trying to say. It would also imply that your MC only cares because his friend was hurt, not because of the general wrongness and injustice of his father's actions. That would make him a less moral protagonist.

However, there is one thing you need to be wary of: you cannot build a character solely on him being _against_ something, as in "against his father". He needs to be also _for_ something - for justice, for people's right to life and safety, etc.

Original score: 5