Of course it's Roger's idea behind the wall. He talks about it in the book with Is There Anybody Out There. He said he had this vision of lobbing bombs into the audience, and the people being blown up were going mad with glee because they were the centre of all the action.

I think the poor bloke had just had enough of people turning up for "that space cadet glow", instead of being interested in the music. After some of the reviews I've read from both Gilmour's, and Waters' recent concerts (concerning the behaviour of people in the crowd) Roger's actions were TOTALLY understandable. Maybe spitting was a little over the top but it was a spontaneaous reaction.

"Just thought you'd like to know"

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Only the very safe,
Can talk about wrong and right.
Of those that are forced to choose,
There's some who will choose to fight.