I don't think "were evil" is actually a secret, most Death Eaters are known - they plead being under the Imperius or otherwise forced to join, but I don't think there is much doubt for who was on which side of the war.
As for telling them publically, it's true it's not the best way to break the news, but I fear attempts to tell them in private could have been ever worse - with rumors propagating in the school, the fact you can't tell them all at once and have to take turns and everyone will see something is going on.
In the current situation, it seems an acceptable way of doing it.
Edit : forgot to refer to the "Voldemort - tried to revive - he summoned Death Eaters and he killed them, stole their blood and life" line of Harry, which is why rumors and doubts were likely already in full motion, justifying a clear public announcement as soon as possible.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 117.
Plans for next chapter release:
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)