The smoking in the diner is an extra layer of grossness on our would-be assassin - by that point in the US, smoking was starting to be looked down on in general.

In that context, the way he stubs the cigarette out speaks to arrogance, he thinks he's got away with it, he's shown them (from Charlie and the rest to whoever would tell him he shouldn't smoke in the diner.)

I suppose it only struck me about Donna because Ginger's instinct was to come to work, while Mrs Landingham and Margeret were at work when they found out, and we saw Mrs Landingham come to the hosptal later. Otherwise, because there's emotional truth at the heart of it - Donna would go where she thought Josh needed her, even if she didn't know how much at the time, I wouldn't have thought anything of it. And none of the other characteurs do. It seems as if they haven't really questioned the Josh-and-Donna of Josh and Donna, just accepted that's how they are.