Who is most responsible for Gatsby's death?
Tom and DaIsy Buchanan are equally to blame for Gatsby's death. Daisy is to blame, firstly, because she drove the car which killed Myrtle Wilson. Secondly because she allows her husband to continue to believe that it was Gatsby who was driving.
Tom is to blame even though he has been misled by his wife; he tells George Wilson that Gatsby was responsible knowing full well what a murderous state of mind Wilson is in; he effectively signs Gatsby's death warrant.
Nick's judgement doesn't differentiate between the two, 'They were careless people, Tom and Daisy - they smashed up things and creatures and then retreated back into their money or their vast carelessness'. No question Nick is thinking of Gatsby and his dream when he says this.