If they weren't exiled they wouldn't have found Drathir's research to be able to go down that path. The only reason they found that research was because they went south because they were banished.
The only "bad" they did was researching the void. Alleria was literally immersed in it. You keep acting like they were performing experiments on the void at the Sunwell. Tell me how someone researching the void quite a ways away with no strict attachment at the time is more dangerous than the original void elf who is able to shift into a void form and has absorbed not just one but two void beings?
My "speculation" is that they were banished for researching the void. Which is what happened in game. Your speculation is that they were somehow concocting experiments near the Sunwell which is what got them kicked, which is complete BS.