purpose of scene with blind old man xing the street

I think you can take it, as someone already put it here, as Bruno's attempt to act natural. Each and every one of us would try to seem like a regular citizen after committing such a crime. However, I think this is also a part of Hitchcock's cynical morale. All of his villains are capable of committing good deeds. So are his heroes -- Guy saving the little boy in the carousel. Bruno's not all bad and neither is Guy all good. This, of course, highlights the main theme of the film regarding the dichotomy of the human nature.

Hitchcock loved irony. He would be delighted that we are tormented by questioning the purpose of some incongruity that appears in his films. So I suppose that his only purpose was to tease. This deed of kindness makes Bruno that more chilling. He can criss-cross from committing murder to a gentle act of delicacy within moments of each other without blinking an eyelid. He is one of my favourite film villains that there has ever been.

It seemed a conscious ploy to me, like Norman Bates' decision not to swat a fly. The act was meant to be protective coloration to me.






"Joey, have you ever been in a Turkish prison?"

Can't cite where I got this but over the years I've either read true stories or have seen in movies - maybe both, the killer who gets some kind of psychic release of tension when he kills. What may be different is that I believe this has usually involves serial killers. From what we know of Bruno, he wasn't a serial killer.
But since Bruno died theres no way to know whether the killer Bruno didn't have the mind of a serial killer and Guys wife may have been just the beginning.
Anyhow, I did wonder whether Hitch was trying to show us this type of damaged brain - hes all worked up before the killing, out of pure nastiness if not evil, he pops a little boys balloon. That was a random target of opportunity where he shows cruelty to a child. After the killing. he comes across another helpless individual, the blind man, and assists him rather than say sneaks up behind him and yells 'Boo', or tripping him, This would have been the equivalent of the earlier kid/balloon scene.
And I'm guessing that Hitch had thought out the character Brunos moves/actions based on what he knew of psychopathic killers.
Bruno pops a kids balloon - nothing in this random act of cruelty would necessitate that Bruno would then follow up with a random act of kindness ... not unless Hitch was thinking along the lines of this 'tension/release' idea. The movie/plot would not have suffered if either of these scenes were never filmed/included so Hitch had to, of course, have a reason to include them - either 1, humor or 2 this tension/release. I will say I thought the kid/balloon scene was, to me, a bit of dark humor - something Hitch used all the time. Had the kid/balloon popping incident occurred prior to the murder, it would have been a foreshadowing/insight into the mind of Bruno ... but we, the audience didn't need this scene at the point it occurred in the story to know Bruno was a potential killer - he had already done the deed. I think it was to show this build up and release ...
Just a thought ....