Maybe he didn't realize it _would_ actually be used.
Supposition:
"Why does the bomb need to be real? That freaks me out."
"We need to make this a real bomb so that if anything goes wrong, you have plausible deniability and can be seen as a victim. They'll take it apart, so if we try to fake anything, it'd be your neck on the line."
"Well, ok, but this is seriously scary."
"Trust me."
Later, at the site, the person with the trigger watches, growing more and more nervous. Thinking to herself:
"What if he cracks? I could go to jail! Oh god, what if there are fingerprints we missed on the thing? This is bad, this is bad. I.... have one chance protect myself. Once they get that bomb off of him, I lose any options. I can control this by pressing the button now. (click)"
