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Fallowers can turn on you in extreme karma changes. For example; if you start shooting BoS soldiers in front of Cross, don't expect her to start disintegrating her brothers and sisters just for your sake.

I have just tested something like this. I enlisted Fawkes as my follower (need 250+ Karma, mine was maxed out at 1000) and then went and killed every resident of Megaton, topping my Karma off at -1000, and Fawkes stayed with me. If I fired him and wanted to re-enlist him, I'd have to regain Karma. ~AEtherchild