Twists and Turns - When your plot takes on a life of its own!

I've been a little stuck with my current story. It's an Alternate History (sci-fi) story that takes place before the Flood. I'm having plenty of trouble developing cultures for a time period where not much is known. On top of that, there is a lot of science involved and lots of extinct creatures. This is the third time my story has taken on a "life of it's own." It can be very disconcerting when this happens. It almost feels like I have multiple personalities.

First time this happened a sub-plot was added with some unsavory characters. I was floored when I was typing and this all came out. It was not in my plan for my story.

Second time. I had a professional author suggest I take one of my flash backs and build it into a prologue or a live action event. I thought I can do that. It's not much longer than a prologue should be. I think I'm into the third chapter of it.

Most recently, aka today, the third event happened. I'm working on the prologue/who knows what. I know where it's going and what will happen. I've even worked out how to connect it with the first part I wrote. I'm trying to add in some of the evil that the Bible says permeated the world during this time period. It has been very hard for me to work that in with the characters and location I have. Today, as I'm changing laundry from the washer to the dryer an idea hit me. I have to write a section from the "evil" point of view. It was kind of scary, since this was nothing I had even remotely considered nor could I see my story going in this direction.

I'm finishing up the "prologue" and then I'll start working on this evil bit. I've already thought of several ideas to throw in. Who knows what the next plot twist will be, or where it will come from!