To raise awareness of a problem in the community.
He even states he doesn't want people to witchhunt.
I actually had to re-watch the Down The Rabbit Hole video because my head is so messed up and confused. And now I have to walk back on what I wrote a while ago. Jagerhunde was indeed the first person to make the connection between Aeris and Sarah. This is important because this means that Aeris = Sarah = Catherine Rain. Meaning that if Jagerhunde is right, then Zack confirming Catherine Rain as Aeris means he is also confirming her as Sarah. Let me try to reiterate in the most non confusing way possible:
1) Jagerhunde reads the Sarah Saga. Identifies Sarah as someone he used to know called Catherine Rain. He posts a link to a Flickr account which contains photos of her.
2) Jagerhunde goes a step further and not only indentifies Sarah as Catherine but also as Aeris from the FF7 House.
3) Zack writes his final update on Demon Sushi, agreeing with Jagerhunde that Catherine is Sarah. What he doesn't explicitly say is that Catherine is Aeris, even though he has Catherine listed among Aeris' aliases (say that 10 times fast).
Why did he never give this final confirmation? Why did he never just come out and say "yes, the girl from the photograph is Aeris"? If he's afraid of feeding a witch hunt then he would have been better off not saying anything. After all the goons from SA believe that all three personalities are the same person. So agreeing that Catherine is Sarah is going to fuel their suspicion that Sarah is Aeris.
