Well, it's never answered directly, so I can only speculate. I think he exited Brood War with a lot of anger, and though that anger probably didn't disappear over the last 4 years, it might have calmed, at least somewhat.
You are correct, that is what I'm saying. Obviously, he couldn't have made that decision before Valerian appears (because that would be illogical). I think he might have changed his stance before in the sense that he is has grown more depressed. It's as if he's looking for a way out that he thinks doesn't exist, even though we later learn that it does.