<p>Eiskatze:
</p><p>Since we don't know what the real motives of the Red Lotus are it is hard to say if they are true villians or well-intentioned extremists. My bet is on the latter. If that is the case, Su could being a member and not contradict her morals, hence not villianize Toph's daughter.
</p><p>My personal theory has been that the Red Lotus want to crown Korra queen of the whole world once they dispose of the current rulers. That is why they have avoided killing any leaders until they get to Korra. If Korra is taken to Ba Sing Sa, they can kill the Earth Queen, save Korra (and get on her good side), while declaring they want her to rule. If this is the case, then Su or anyone would have good reason to join the Red Lotus, they want the avatar to stop being a bystander and take a more hands on approach. Remember, Zaheer said that overthrowing current rulers was just half of his plan to "plant the seeds for a new world". Why would they need Korra for it?
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