Ummmm... I never did. Mussolini may have been a socialist, though he very much later disclaimed that ideology and most of his supporters were very firm anti-socialist
before WWI and this was true for nearly all fascist movements (with the possible exception of Romania where the Iron Guard supporters tended to be working class and to lesser extent, the Hungarian Arrow Cross but that might be due to all anti-Horthyist parties being outlawed or put into mild mannered acceptance) After Mussolini every other fascist movement saw socialists of all varieties as their number one enemy.
I have never once claimed that Socialist nor Fascism could not be linked, indeed proto-ideologies linking the two existed before WWI (The Eugenics of HG Wells' and others comes to mind) and both ideologies fundamentally against the arbitrary liberal capitalism that existed prior to 1914 and which has seen to have been responsible for the war post-1918 (this view acclerated after the Great Depression). And there is no doubt that fascism borrowed alot for communism in its rhetoric and in its messianic nature (IIRC Hitler made 1st May a bank Holiday when arriving into power). However both ideologies were different in their intellectual origins and their ideological purpose, as seen above*. I am merely disputing the fascism=left because it meant a big government hypothesis.
* - (Though of course when talking about the USSR, et al you have a point. But in most of Western Europe and the rest of the 1st world pro-soviets tended to make up a minority of socialists, only in Germany did they really have a shot at actual power. Not even in Italy, unless you count an increase in street fighting as a revolutionary concern.)