Right from the horses mouth... transcript of the national radio program that Roger Waters and Jim Ladd did about the Wall.
http://www.pink-floyd.org/artint/119.htm
[Waiting For The Worms]
RW: And then he keeps flipping into the other persona which is the raving, facist persona that he has adopted. I could explain one thing and that is that all that shouting, the bullhorn stuff is actually describing a march from a place in south London. It's a heavily black populated area of south London where the National Front is particularly active. And it describes a march from a place called Brixton Town Hall and it just describes the roads and things and which bridge they come over and where they're going. And they're going towards Hyde Park Corner to have a rally in Hyde Park. And at the end of it, I don't know if can hear, they're saying hammer. And that's another thing. On side four, the audience who start off in between 'The Show Must Go On' and 'In The Flesh'...you can hear them chanting Pink Floyd, but slowly that gets taken over by Hammer. The idea was for a rock show to turn into a rally.
JL: These last few lines in 'Stop': "Because I have to know / Have I been guilty all this time." Is Pink asking himself, 'Am I a contributor to this very Hell that I have made for myself?'
RW: Yeah. Exactly that.
[Stop]
JL: And what's your judgement on that?
RW: Well...it all comes out in the next song.
JL: Which is 'The Trial.'
[The Trial begins]
JL: And here you bring back the characters of the school master, the wife, the mother.
RW: And he has been obviously. We have been. We did do Aneheim stadium last time where the first people in the audience are 120 yards away because they won't let you on the infield. A dreadful place to do a rock show. The only reason for doing it is money. The defense of doing huge gigs like that, where nobody can really hear anything or see anything, is 'well gosh, we're so popular...there are so many people who want to come see us that we have to do these very large venues in order to accomadate them.' Which is a very nice gently out, but the reason that we all do it is for the bucks. And don't let any rock and roller tell you different because it ain't true. So, he decides that he has, we have. All those big stadium gigs. As it's himself that's trying himself. The worst thing that he thinks can happen to him is that he should expose himself, and his fears, and his feelings, and anything to make himself vunerable.
JL: Now wait a minute. You say that he's judging himself?
RW: Yeah. This is him hallucinating. This is him breaking down.