Selfish in general seems to be a derogatory term. We tend to think a person is selfish if he/she is self-centered at the expense of others. The paragraph it self says that Roark is selfish in a 'positive sense'. Keeping up to ones own ideas I believe is not selfish. If it is then even Gautam Buddha can be called selfish as he also went in search for truth, to satisfy his own ideas. Having a self is not being selfish. If the paragraph calls him selfish, then may be the term we must use in a derogatory sense should be 'SELFIST' (although there is no such word).
People who wanted to modify Roark's design, can then be called Selfists, as they are trying to gain something at the expense of Roark. Infact, Peter Keating kept on doing that all along the book. Peter can be called as a Selfist.
Roark never acquired anything at the expense of others. Every person who creates something considers it as his own child. One cannot let anyone tamper with its originality because that's what it makes it so special for you, that you created it.
(there's lots more to write, but won't bore you with too many paragraphs.

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