Full Idea
'Right' does and can mean nothing but 'cause of a good result', and is thus identical with 'useful', whence it follows that the end always will justify the means.
Gist of Idea
'Right' means 'cause of good result' (hence 'useful'), so the end does justify the means
Source
G.E. Moore (Principia Ethica [1903], §089)
Book Reference
Moore,G.E.: 'Principia Ethica' [CUP 1980], p.147
A Reaction
Of course, Moore does not identify utility with pleasure, as his notion of what is good concerns fairly Platonic ideals. Would Stalin's murders have been right if Russia were now the happiest nation on Earth?