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Source: https://writers.stackexchange.com/a/34252
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/34252
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

The main problem will likely be that your readers should like your protagonist - you want them to read a whole series with him as the main character after all. If the character you like suddenly without any warning turns into the super bad guy they will ask themselves "What happened?!" and they may not like that.

Furthermore you have to be careful about how you present the protagonist-turned-antagonist. If you are for example writing from the first person perspective it should be obvious that he sometimes has _evil_ plans. If not you should think very carefully about what it was that made him turn to the _dark side_. What was the reason for him to suddenly be one of the bad guys? Is it maybe one of his main character traits, like never breaking a promise or bringing someone he loved back from the dead or because _it's better to have a good dictator than a bad democracy_, ...? Why would he do that and why did he not do something like that when he was the protagonist?

But it's possible - with enough foreshadowing and knowing that it _might_ turn away people that want their favourite to stay their favourite.

* * *

If you already showed that your initial protagonist doesn't like your later protagonist(s) and that he is not afraid of going... unusual... paths to achieve his goals it might even feel normal or like the expected climax of the story - people were expecting that _something_ should happen. You should make it feel like the books of your first "protagonist" are missing something. The final fight. Like it's not over yet and they still have some business to take care of with the other(s). Then your readers will anticipate it and will not be surprised or even turned away by this. Still, a protagonist changing to the dark side may feel a bit weird for some people and there might still be some who won't accept that ending - you can't please everyone.

Original score: 3