I think he was going to plead guilty the entire time
I don't actually think he ever really wanted to fight this thing. The evidence he left behind (to the feds...) and lack of plan post-incident show that, as well as his statements in PA. In my opinion, he was forced to fight it because the unexpected happened: the death penalty and two trials for one crime. If it was only the feds that tried this, I would bet he would have accepted either a plea or just pled guilty sooner. I never really understood why someone would do something like this with purpose and intent, and then plead "not guilty" and take it to trial. He never once tried to deny it and even in his few statements, has made it pretty clear he did this.
Long story short, I think this was his plan all along and now that he got rid of the DP and potentially state trial, he's likely at peace living with what he's done.