Here's my theory on this whole story
First of all. I love SC1. SC2 and 3 were good for entertainment and fun. Pamela Springsteen did a remarkable job in these sequels. However, no one can top Felissa Rose. She played the perfect Angela, and she totally creeped me out.
So here's my theory. I think Angela Johnson was former patient from the institution where Angela Baker was. They both became friends during their time spent in the hospital together. And I believe Angela Baker opened up to her and told Angela Johnson her past history including the murders and her reasoning behind it.
Therefore shortly before Angela Johnson's release, she became deeply obsessed and wanted to carry on Angela Bakers work. I know, it's just a theory. But a very cool one. Because if you've seen Return To Sleepaway Camp, Angela Baker's cousin Ricky mentions to the cop that she's been locked up in the hospital for the past 20 years. So there's no way this could be the "Real Angela" in SC2 or 3, just an obsessed copycat.
And I heard Sleepaway Camp Reunion 3-D is in the works. With the return of Felissa Rose as the real Angela. I hope it comes out soon. There's so much more they can do with this story.