I have written about the issue at some lenght here (I have not actually wrapped up everything as I would have liked, but it should address your question).

In short - while the idea of an "opportunistic" Mussolini used to enjoy a certain popularity back in the day - more recent works (if by recent we mean the 1960s with De Felice's monograph), including those of Gentile, Milza, Griffin (more precise references in the answer linked), have come to the conclusion (which I completely agree with) that: a) Mussolini's ideological change was less stark that it might appear, in so far as it occurred throught a long period of time (indeed since his early socialist frequentations), across a momentous series of events (the Great War, the Russian Revolution, etc.), wasn't exclusive to him (many other intellectual and political figures experiences a similar, if not identical trajectory) and took origin from a position that had always had more in common with a certain "radicalism" and "intransigentism" of activist character, than from an established foundation of "far-left socialism" (while somewhat reductionist if you look at the general picture, Sternhell's discussion of this form of third-stage revisionism of Marxism is quite helpful). And b) Mussolini certainly was discredited among socialists by his lack of conviction. In fact he continued to be subject to the accusation of being a mere opportunist and "bourgeois shill" (at least) as long as this specific accusation remained the worst of his sins from a socialist perspective. But you can't expect the claim that socialism and internationalism had failed, that they had died with the Great War and that Bolshevism was "proof that they were gone for good", to be equally rejected by those anti-Bolshevik groups and activists who would rather find a welcome confirmation in their position being professed by one who used to be a prominent socialist leader.