Imagination (or should I say expectations) is one thing, and reality is another. E&D could have planned in the greatest detail, but a lot of things are unpredictable, as is their behavior induced by their feelings in such an extreme situation as killing. I find that the vast majority of people can’t even imagine how they would feel in such a situation, especially when the initial euphoria subsides and reality kicks in.
For example, there was one Columbine-inspired case in my country a few years ago (it is also the only one). The high school student went to his old elementary school, threw Molotov cocktails, but when he was about to shoot the teacher, he simply realized at that moment that it was just "not him" and handed over the weapon. Sure, he was punished and ended up in a facility, but what I want to say is that it’s hard to even imagine what it’s like to find yourself in the situation of a mass murder perpetrator at all. We’re all used to movies and games, but if we really had to shoot someone, I think it would traumatize any at least "normal" person, and by my criteria, E&D were conscious and reasonable individuals who just made a really bad decision.
So, I think there were definitely moments when they realized what they were doing and the enjoyment was gone. That would largely explain their aimless wandering the hallways before committing suicide.
(Of course, I know their plan failed because the bombs did not detonate and that it was a turn off right from the start.)