It was done with forethought.
The fact is we can suppose, but we cannot know if there was forethought. The jury thought so, and they had access to the info we don't, so I tend to think you're right.
But I also think he could
be still panic-stricken and not show it. See, I've seen people walk exactly as calmly as he did, and pick up the last pencil on their desks when a bombing was well underway. The mind is not a simple device.
I agree that maybe- just maybe- he would have walked free if it wasn't for his lying.