Lizz
They're saying the f/o was having normal conversations at the start of the flight, then on the brief for the decent he's 'laconic'. Forgive me if I'm wrong but unless the captain had said before the brief that he was going to the toilet afterwards, f/o wouldn't have known he would be left alone thus not realising until after the brief that he could do whatever he did so his bluntness surely would have nothing to do with it?
I doubt for one minute it was a spur of moment decision to do what he did, this kind of act would have been planned a long time before and he would have been waiting for the right opportunity.