Dear Roger
Roger Waters was the one most affected emotionally about Syd's demise. Listen to the last bit of Dark Side and they basically wrote the the whole of Wish You Were Here about him. "Shine On You Crazy Diamond?"
(Dr Teeth Who asked you, you fuckin' hobbit?, Wed 10 Mar 2010, 2:54, 1 reply)

He was
motivated by a degree of guilt, I believe. But not initially. I remember reading an account of how after the big acid trip they wanted Syd to be an active member of the band, but he was so erratic that they couldn't count on him. On the way to a gig someone said "Hang on, aren't we going go pick up Syd?" and Roger Waters said "Oh let's don't," and they went on without him.

I think that Waters was hit with a lot of guilt in the aftermath, but at the time he couldn't be bothered.

To be fair, I can't really blame him for it- I probably would have done the same. But Gilmour was the one who ultimately cared for Syd, so I have nothing but great admiration for the man.
( Wanderer, Wed 10 Mar 2010, 5:02, closed)

Especially
As shortly after Barret was fired, he would turn up at gigs just to glare at and insult Gilmour for taking his place.
( MatJ LOOK, Wed 10 Mar 2010, 16:13, closed)

Even stranger than that
When they were recording Wish You Were Here there was one day when this balding guy showed up in the studio and just sat to one side, staring at them as they worked, then left without saying anything. It wasn't until he had gone that Rick Wright suddenly turned to the others and said, "Oh shit, was that Syd?!?"

It rattled them a bit for the rest of the day.
( Wanderer, Wed 10 Mar 2010, 23:52, closed)